//===---------------------------------------------------------------------===// // Random ideas for the X86 backend. //===---------------------------------------------------------------------===// //===---------------------------------------------------------------------===// CodeGen/X86/lea-3.ll:test3 should be a single LEA, not a shift/move. The X86 backend knows how to three-addressify this shift, but it appears the register allocator isn't even asking it to do so in this case. We should investigate why this isn't happening, it could have significant impact on other important cases for X86 as well. //===---------------------------------------------------------------------===// This should be one DIV/IDIV instruction, not a libcall: unsigned test(unsigned long long X, unsigned Y) { return X/Y; } This can be done trivially with a custom legalizer. What about overflow though? http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14224 //===---------------------------------------------------------------------===// Improvements to the multiply -> shift/add algorithm: http://gcc.gnu.org/ml/gcc-patches/2004-08/msg01590.html //===---------------------------------------------------------------------===// Improve code like this (occurs fairly frequently, e.g. in LLVM): long long foo(int x) { return 1LL << x; } http://gcc.gnu.org/ml/gcc-patches/2004-09/msg01109.html http://gcc.gnu.org/ml/gcc-patches/2004-09/msg01128.html http://gcc.gnu.org/ml/gcc-patches/2004-09/msg01136.html Another useful one would be ~0ULL >> X and ~0ULL << X. One better solution for 1LL << x is: xorl %eax, %eax xorl %edx, %edx testb $32, %cl sete %al setne %dl sall %cl, %eax sall %cl, %edx But that requires good 8-bit subreg support. Also, this might be better. It's an extra shift, but it's one instruction shorter, and doesn't stress 8-bit subreg support. (From http://gcc.gnu.org/ml/gcc-patches/2004-09/msg01148.html, but without the unnecessary and.) movl %ecx, %eax shrl $5, %eax movl %eax, %edx xorl $1, %edx sall %cl, %eax sall %cl. %edx 64-bit shifts (in general) expand to really bad code. Instead of using cmovs, we should expand to a conditional branch like GCC produces. //===---------------------------------------------------------------------===// Compile this: _Bool f(_Bool a) { return a!=1; } into: movzbl %dil, %eax xorl $1, %eax ret (Although note that this isn't a legal way to express the code that llvm-gcc currently generates for that function.) //===---------------------------------------------------------------------===// Some isel ideas: 1. Dynamic programming based approach when compile time if not an issue. 2. Code duplication (addressing mode) during isel. 3. Other ideas from "Register-Sensitive Selection, Duplication, and Sequencing of Instructions". 4. Scheduling for reduced register pressure. E.g. "Minimum Register Instruction Sequence Problem: Revisiting Optimal Code Generation for DAGs" and other related papers. http://citeseer.ist.psu.edu/govindarajan01minimum.html //===---------------------------------------------------------------------===// Should we promote i16 to i32 to avoid partial register update stalls? //===---------------------------------------------------------------------===// Leave any_extend as pseudo instruction and hint to register allocator. Delay codegen until post register allocation. Note. any_extend is now turned into an INSERT_SUBREG. We still need to teach the coalescer how to deal with it though. //===---------------------------------------------------------------------===// It appears icc use push for parameter passing. Need to investigate. //===---------------------------------------------------------------------===// Only use inc/neg/not instructions on processors where they are faster than add/sub/xor. They are slower on the P4 due to only updating some processor flags. //===---------------------------------------------------------------------===// The instruction selector sometimes misses folding a load into a compare. The pattern is written as (cmp reg, (load p)). Because the compare isn't commutative, it is not matched with the load on both sides. The dag combiner should be made smart enough to cannonicalize the load into the RHS of a compare when it can invert the result of the compare for free. //===---------------------------------------------------------------------===// How about intrinsics? An example is: *res = _mm_mulhi_epu16(*A, _mm_mul_epu32(*B, *C)); compiles to pmuludq (%eax), %xmm0 movl 8(%esp), %eax movdqa (%eax), %xmm1 pmulhuw %xmm0, %xmm1 The transformation probably requires a X86 specific pass or a DAG combiner target specific hook. //===---------------------------------------------------------------------===// In many cases, LLVM generates code like this: _test: movl 8(%esp), %eax cmpl %eax, 4(%esp) setl %al movzbl %al, %eax ret on some processors (which ones?), it is more efficient to do this: _test: movl 8(%esp), %ebx xor %eax, %eax cmpl %ebx, 4(%esp) setl %al ret Doing this correctly is tricky though, as the xor clobbers the flags. //===---------------------------------------------------------------------===// We should generate bts/btr/etc instructions on targets where they are cheap or when codesize is important. e.g., for: void setbit(int *target, int bit) { *target |= (1 << bit); } void clearbit(int *target, int bit) { *target &= ~(1 << bit); } //===---------------------------------------------------------------------===// Instead of the following for memset char*, 1, 10: movl $16843009, 4(%edx) movl $16843009, (%edx) movw $257, 8(%edx) It might be better to generate movl $16843009, %eax movl %eax, 4(%edx) movl %eax, (%edx) movw al, 8(%edx) when we can spare a register. It reduces code size. //===---------------------------------------------------------------------===// Evaluate what the best way to codegen sdiv X, (2^C) is. For X/8, we currently get this: define i32 @test1(i32 %X) { %Y = sdiv i32 %X, 8 ret i32 %Y } _test1: movl 4(%esp), %eax movl %eax, %ecx sarl $31, %ecx shrl $29, %ecx addl %ecx, %eax sarl $3, %eax ret GCC knows several different ways to codegen it, one of which is this: _test1: movl 4(%esp), %eax cmpl $-1, %eax leal 7(%eax), %ecx cmovle %ecx, %eax sarl $3, %eax ret which is probably slower, but it's interesting at least :) //===---------------------------------------------------------------------===// We are currently lowering large (1MB+) memmove/memcpy to rep/stosl and rep/movsl We should leave these as libcalls for everything over a much lower threshold, since libc is hand tuned for medium and large mem ops (avoiding RFO for large stores, TLB preheating, etc) //===---------------------------------------------------------------------===// Optimize this into something reasonable: x * copysign(1.0, y) * copysign(1.0, z) //===---------------------------------------------------------------------===// Optimize copysign(x, *y) to use an integer load from y. //===---------------------------------------------------------------------===// %X = weak global int 0 void %foo(int %N) { %N = cast int %N to uint %tmp.24 = setgt int %N, 0 br bool %tmp.24, label %no_exit, label %return no_exit: %indvar = phi uint [ 0, %entry ], [ %indvar.next, %no_exit ] %i.0.0 = cast uint %indvar to int volatile store int %i.0.0, int* %X %indvar.next = add uint %indvar, 1 %exitcond = seteq uint %indvar.next, %N br bool %exitcond, label %return, label %no_exit return: ret void } compiles into: .text .align 4 .globl _foo _foo: movl 4(%esp), %eax cmpl $1, %eax jl LBB_foo_4 # return LBB_foo_1: # no_exit.preheader xorl %ecx, %ecx LBB_foo_2: # no_exit movl L_X$non_lazy_ptr, %edx movl %ecx, (%edx) incl %ecx cmpl %eax, %ecx jne LBB_foo_2 # no_exit LBB_foo_3: # return.loopexit LBB_foo_4: # return ret We should hoist "movl L_X$non_lazy_ptr, %edx" out of the loop after remateralization is implemented. This can be accomplished with 1) a target dependent LICM pass or 2) makeing SelectDAG represent the whole function. //===---------------------------------------------------------------------===// The following tests perform worse with LSR: lambda, siod, optimizer-eval, ackermann, hash2, nestedloop, strcat, and Treesor. //===---------------------------------------------------------------------===// We are generating far worse code than gcc: volatile short X, Y; void foo(int N) { int i; for (i = 0; i < N; i++) { X = i; Y = i*4; } } LBB1_1: # entry.bb_crit_edge xorl %ecx, %ecx xorw %dx, %dx LBB1_2: # bb movl L_X$non_lazy_ptr, %esi movw %cx, (%esi) movl L_Y$non_lazy_ptr, %esi movw %dx, (%esi) addw $4, %dx incl %ecx cmpl %eax, %ecx jne LBB1_2 # bb vs. xorl %edx, %edx movl L_X$non_lazy_ptr-"L00000000001$pb"(%ebx), %esi movl L_Y$non_lazy_ptr-"L00000000001$pb"(%ebx), %ecx L4: movw %dx, (%esi) leal 0(,%edx,4), %eax movw %ax, (%ecx) addl $1, %edx cmpl %edx, %edi jne L4 This is due to the lack of post regalloc LICM. //===---------------------------------------------------------------------===// Teach the coalescer to coalesce vregs of different register classes. e.g. FR32 / FR64 to VR128. //===---------------------------------------------------------------------===// mov $reg, 48(%esp) ... leal 48(%esp), %eax mov %eax, (%esp) call _foo Obviously it would have been better for the first mov (or any op) to store directly %esp[0] if there are no other uses. //===---------------------------------------------------------------------===// Adding to the list of cmp / test poor codegen issues: int test(__m128 *A, __m128 *B) { if (_mm_comige_ss(*A, *B)) return 3; else return 4; } _test: movl 8(%esp), %eax movaps (%eax), %xmm0 movl 4(%esp), %eax movaps (%eax), %xmm1 comiss %xmm0, %xmm1 setae %al movzbl %al, %ecx movl $3, %eax movl $4, %edx cmpl $0, %ecx cmove %edx, %eax ret Note the setae, movzbl, cmpl, cmove can be replaced with a single cmovae. There are a number of issues. 1) We are introducing a setcc between the result of the intrisic call and select. 2) The intrinsic is expected to produce a i32 value so a any extend (which becomes a zero extend) is added. We probably need some kind of target DAG combine hook to fix this. //===---------------------------------------------------------------------===// We generate significantly worse code for this than GCC: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21150 http://gcc.gnu.org/bugzilla/attachment.cgi?id=8701 There is also one case we do worse on PPC. //===---------------------------------------------------------------------===// If shorter, we should use things like: movzwl %ax, %eax instead of: andl $65535, %EAX The former can also be used when the two-addressy nature of the 'and' would require a copy to be inserted (in X86InstrInfo::convertToThreeAddress). //===---------------------------------------------------------------------===// Another instruction selector deficiency: void %bar() { %tmp = load int (int)** %foo %tmp = tail call int %tmp( int 3 ) ret void } _bar: subl $12, %esp movl L_foo$non_lazy_ptr, %eax movl (%eax), %eax call *%eax addl $12, %esp ret The current isel scheme will not allow the load to be folded in the call since the load's chain result is read by the callseq_start. //===---------------------------------------------------------------------===// For this: int test(int a) { return a * 3; } We currently emits imull $3, 4(%esp), %eax Perhaps this is what we really should generate is? Is imull three or four cycles? Note: ICC generates this: movl 4(%esp), %eax leal (%eax,%eax,2), %eax The current instruction priority is based on pattern complexity. The former is more "complex" because it folds a load so the latter will not be emitted. Perhaps we should use AddedComplexity to give LEA32r a higher priority? We should always try to match LEA first since the LEA matching code does some estimate to determine whether the match is profitable. However, if we care more about code size, then imull is better. It's two bytes shorter than movl + leal. //===---------------------------------------------------------------------===// __builtin_ffs codegen is messy. int ffs_(unsigned X) { return __builtin_ffs(X); } llvm produces: ffs_: movl 4(%esp), %ecx bsfl %ecx, %eax movl $32, %edx cmove %edx, %eax incl %eax xorl %edx, %edx testl %ecx, %ecx cmove %edx, %eax ret vs gcc: _ffs_: movl $-1, %edx bsfl 4(%esp), %eax cmove %edx, %eax addl $1, %eax ret Another example of __builtin_ffs (use predsimplify to eliminate a select): int foo (unsigned long j) { if (j) return __builtin_ffs (j) - 1; else return 0; } //===---------------------------------------------------------------------===// It appears gcc place string data with linkonce linkage in .section __TEXT,__const_coal,coalesced instead of .section __DATA,__const_coal,coalesced. Take a look at darwin.h, there are other Darwin assembler directives that we do not make use of. //===---------------------------------------------------------------------===// define i32 @foo(i32* %a, i32 %t) { entry: br label %cond_true cond_true: ; preds = %cond_true, %entry %x.0.0 = phi i32 [ 0, %entry ], [ %tmp9, %cond_true ] ; [#uses=3] %t_addr.0.0 = phi i32 [ %t, %entry ], [ %tmp7, %cond_true ] ; [#uses=1] %tmp2 = getelementptr i32* %a, i32 %x.0.0 ; [#uses=1] %tmp3 = load i32* %tmp2 ; [#uses=1] %tmp5 = add i32 %t_addr.0.0, %x.0.0 ; [#uses=1] %tmp7 = add i32 %tmp5, %tmp3 ; [#uses=2] %tmp9 = add i32 %x.0.0, 1 ; [#uses=2] %tmp = icmp sgt i32 %tmp9, 39 ; [#uses=1] br i1 %tmp, label %bb12, label %cond_true bb12: ; preds = %cond_true ret i32 %tmp7 } is pessimized by -loop-reduce and -indvars //===---------------------------------------------------------------------===// u32 to float conversion improvement: float uint32_2_float( unsigned u ) { float fl = (int) (u & 0xffff); float fh = (int) (u >> 16); fh *= 0x1.0p16f; return fh + fl; } 00000000 subl $0x04,%esp 00000003 movl 0x08(%esp,1),%eax 00000007 movl %eax,%ecx 00000009 shrl $0x10,%ecx 0000000c cvtsi2ss %ecx,%xmm0 00000010 andl $0x0000ffff,%eax 00000015 cvtsi2ss %eax,%xmm1 00000019 mulss 0x00000078,%xmm0 00000021 addss %xmm1,%xmm0 00000025 movss %xmm0,(%esp,1) 0000002a flds (%esp,1) 0000002d addl $0x04,%esp 00000030 ret //===---------------------------------------------------------------------===// When using fastcc abi, align stack slot of argument of type double on 8 byte boundary to improve performance. //===---------------------------------------------------------------------===// Codegen: int f(int a, int b) { if (a == 4 || a == 6) b++; return b; } as: or eax, 2 cmp eax, 6 jz label //===---------------------------------------------------------------------===// GCC's ix86_expand_int_movcc function (in i386.c) has a ton of interesting simplifications for integer "x cmp y ? a : b". For example, instead of: int G; void f(int X, int Y) { G = X < 0 ? 14 : 13; } compiling to: _f: movl $14, %eax movl $13, %ecx movl 4(%esp), %edx testl %edx, %edx cmovl %eax, %ecx movl %ecx, _G ret it could be: _f: movl 4(%esp), %eax sarl $31, %eax notl %eax addl $14, %eax movl %eax, _G ret etc. Another is: int usesbb(unsigned int a, unsigned int b) { return (a < b ? -1 : 0); } to: _usesbb: movl 8(%esp), %eax cmpl %eax, 4(%esp) sbbl %eax, %eax ret instead of: _usesbb: xorl %eax, %eax movl 8(%esp), %ecx cmpl %ecx, 4(%esp) movl $4294967295, %ecx cmovb %ecx, %eax ret //===---------------------------------------------------------------------===// Currently we don't have elimination of redundant stack manipulations. Consider the code: int %main() { entry: call fastcc void %test1( ) call fastcc void %test2( sbyte* cast (void ()* %test1 to sbyte*) ) ret int 0 } declare fastcc void %test1() declare fastcc void %test2(sbyte*) This currently compiles to: subl $16, %esp call _test5 addl $12, %esp subl $16, %esp movl $_test5, (%esp) call _test6 addl $12, %esp The add\sub pair is really unneeded here. //===---------------------------------------------------------------------===// Consider the expansion of: define i32 @test3(i32 %X) { %tmp1 = urem i32 %X, 255 ret i32 %tmp1 } Currently it compiles to: ... movl $2155905153, %ecx movl 8(%esp), %esi movl %esi, %eax mull %ecx ... This could be "reassociated" into: movl $2155905153, %eax movl 8(%esp), %ecx mull %ecx to avoid the copy. In fact, the existing two-address stuff would do this except that mul isn't a commutative 2-addr instruction. I guess this has to be done at isel time based on the #uses to mul? //===---------------------------------------------------------------------===// Make sure the instruction which starts a loop does not cross a cacheline boundary. This requires knowning the exact length of each machine instruction. That is somewhat complicated, but doable. Example 256.bzip2: In the new trace, the hot loop has an instruction which crosses a cacheline boundary. In addition to potential cache misses, this can't help decoding as I imagine there has to be some kind of complicated decoder reset and realignment to grab the bytes from the next cacheline. 532 532 0x3cfc movb (1809(%esp, %esi), %bl <<<--- spans 2 64 byte lines 942 942 0x3d03 movl %dh, (1809(%esp, %esi) 937 937 0x3d0a incl %esi 3 3 0x3d0b cmpb %bl, %dl 27 27 0x3d0d jnz 0x000062db //===---------------------------------------------------------------------===// In c99 mode, the preprocessor doesn't like assembly comments like #TRUNCATE. //===---------------------------------------------------------------------===// This could be a single 16-bit load. int f(char *p) { if ((p[0] == 1) & (p[1] == 2)) return 1; return 0; } //===---------------------------------------------------------------------===// We should inline lrintf and probably other libc functions. //===---------------------------------------------------------------------===// Start using the flags more. For example, compile: int add_zf(int *x, int y, int a, int b) { if ((*x += y) == 0) return a; else return b; } to: addl %esi, (%rdi) movl %edx, %eax cmovne %ecx, %eax ret instead of: _add_zf: addl (%rdi), %esi movl %esi, (%rdi) testl %esi, %esi cmove %edx, %ecx movl %ecx, %eax ret and: int add_zf(int *x, int y, int a, int b) { if ((*x + y) < 0) return a; else return b; } to: add_zf: addl (%rdi), %esi movl %edx, %eax cmovns %ecx, %eax ret instead of: _add_zf: addl (%rdi), %esi testl %esi, %esi cmovs %edx, %ecx movl %ecx, %eax ret //===---------------------------------------------------------------------===// These two functions have identical effects: unsigned int f(unsigned int i, unsigned int n) {++i; if (i == n) ++i; return i;} unsigned int f2(unsigned int i, unsigned int n) {++i; i += i == n; return i;} We currently compile them to: _f: movl 4(%esp), %eax movl %eax, %ecx incl %ecx movl 8(%esp), %edx cmpl %edx, %ecx jne LBB1_2 #UnifiedReturnBlock LBB1_1: #cond_true addl $2, %eax ret LBB1_2: #UnifiedReturnBlock movl %ecx, %eax ret _f2: movl 4(%esp), %eax movl %eax, %ecx incl %ecx cmpl 8(%esp), %ecx sete %cl movzbl %cl, %ecx leal 1(%ecx,%eax), %eax ret both of which are inferior to GCC's: _f: movl 4(%esp), %edx leal 1(%edx), %eax addl $2, %edx cmpl 8(%esp), %eax cmove %edx, %eax ret _f2: movl 4(%esp), %eax addl $1, %eax xorl %edx, %edx cmpl 8(%esp), %eax sete %dl addl %edx, %eax ret //===---------------------------------------------------------------------===// This code: void test(int X) { if (X) abort(); } is currently compiled to: _test: subl $12, %esp cmpl $0, 16(%esp) jne LBB1_1 addl $12, %esp ret LBB1_1: call L_abort$stub It would be better to produce: _test: subl $12, %esp cmpl $0, 16(%esp) jne L_abort$stub addl $12, %esp ret This can be applied to any no-return function call that takes no arguments etc. Alternatively, the stack save/restore logic could be shrink-wrapped, producing something like this: _test: cmpl $0, 4(%esp) jne LBB1_1 ret LBB1_1: subl $12, %esp call L_abort$stub Both are useful in different situations. Finally, it could be shrink-wrapped and tail called, like this: _test: cmpl $0, 4(%esp) jne LBB1_1 ret LBB1_1: pop %eax # realign stack. call L_abort$stub Though this probably isn't worth it. //===---------------------------------------------------------------------===// We need to teach the codegen to convert two-address INC instructions to LEA when the flags are dead (likewise dec). For example, on X86-64, compile: int foo(int A, int B) { return A+1; } to: _foo: leal 1(%edi), %eax ret instead of: _foo: incl %edi movl %edi, %eax ret Another example is: ;; X's live range extends beyond the shift, so the register allocator ;; cannot coalesce it with Y. Because of this, a copy needs to be ;; emitted before the shift to save the register value before it is ;; clobbered. However, this copy is not needed if the register ;; allocator turns the shift into an LEA. This also occurs for ADD. ; Check that the shift gets turned into an LEA. ; RUN: llvm-as < %s | llc -march=x86 -x86-asm-syntax=intel | \ ; RUN: not grep {mov E.X, E.X} @G = external global i32 ; [#uses=3] define i32 @test1(i32 %X, i32 %Y) { %Z = add i32 %X, %Y ; [#uses=1] volatile store i32 %Y, i32* @G volatile store i32 %Z, i32* @G ret i32 %X } define i32 @test2(i32 %X) { %Z = add i32 %X, 1 ; [#uses=1] volatile store i32 %Z, i32* @G ret i32 %X } //===---------------------------------------------------------------------===// Sometimes it is better to codegen subtractions from a constant (e.g. 7-x) with a neg instead of a sub instruction. Consider: int test(char X) { return 7-X; } we currently produce: _test: movl $7, %eax movsbl 4(%esp), %ecx subl %ecx, %eax ret We would use one fewer register if codegen'd as: movsbl 4(%esp), %eax neg %eax add $7, %eax ret Note that this isn't beneficial if the load can be folded into the sub. In this case, we want a sub: int test(int X) { return 7-X; } _test: movl $7, %eax subl 4(%esp), %eax ret //===---------------------------------------------------------------------===// Leaf functions that require one 4-byte spill slot have a prolog like this: _foo: pushl %esi subl $4, %esp ... and an epilog like this: addl $4, %esp popl %esi ret It would be smaller, and potentially faster, to push eax on entry and to pop into a dummy register instead of using addl/subl of esp. Just don't pop into any return registers :) //===---------------------------------------------------------------------===// The X86 backend should fold (branch (or (setcc, setcc))) into multiple branches. We generate really poor code for: double testf(double a) { return a == 0.0 ? 0.0 : (a > 0.0 ? 1.0 : -1.0); } For example, the entry BB is: _testf: subl $20, %esp pxor %xmm0, %xmm0 movsd 24(%esp), %xmm1 ucomisd %xmm0, %xmm1 setnp %al sete %cl testb %cl, %al jne LBB1_5 # UnifiedReturnBlock LBB1_1: # cond_true it would be better to replace the last four instructions with: jp LBB1_1 je LBB1_5 LBB1_1: We also codegen the inner ?: into a diamond: cvtss2sd LCPI1_0(%rip), %xmm2 cvtss2sd LCPI1_1(%rip), %xmm3 ucomisd %xmm1, %xmm0 ja LBB1_3 # cond_true LBB1_2: # cond_true movapd %xmm3, %xmm2 LBB1_3: # cond_true movapd %xmm2, %xmm0 ret We should sink the load into xmm3 into the LBB1_2 block. This should be pretty easy, and will nuke all the copies. //===---------------------------------------------------------------------===// This: #include inline std::pair full_add(unsigned a, unsigned b) { return std::make_pair(a + b, a + b < a); } bool no_overflow(unsigned a, unsigned b) { return !full_add(a, b).second; } Should compile to: _Z11no_overflowjj: addl %edi, %esi setae %al ret FIXME: That code looks wrong; bool return is normally defined as zext. on x86-64, not: __Z11no_overflowjj: addl %edi, %esi cmpl %edi, %esi setae %al movzbl %al, %eax ret //===---------------------------------------------------------------------===// Re-materialize MOV32r0 etc. with xor instead of changing them to moves if the condition register is dead. xor reg reg is shorter than mov reg, #0. //===---------------------------------------------------------------------===// We aren't matching RMW instructions aggressively enough. Here's a reduced testcase (more in PR1160): define void @test(i32* %huge_ptr, i32* %target_ptr) { %A = load i32* %huge_ptr ; [#uses=1] %B = load i32* %target_ptr ; [#uses=1] %C = or i32 %A, %B ; [#uses=1] store i32 %C, i32* %target_ptr ret void } $ llvm-as < t.ll | llc -march=x86-64 _test: movl (%rdi), %eax orl (%rsi), %eax movl %eax, (%rsi) ret That should be something like: _test: movl (%rdi), %eax orl %eax, (%rsi) ret //===---------------------------------------------------------------------===// The following code: bb114.preheader: ; preds = %cond_next94 %tmp231232 = sext i16 %tmp62 to i32 ; [#uses=1] %tmp233 = sub i32 32, %tmp231232 ; [#uses=1] %tmp245246 = sext i16 %tmp65 to i32 ; [#uses=1] %tmp252253 = sext i16 %tmp68 to i32 ; [#uses=1] %tmp254 = sub i32 32, %tmp252253 ; [#uses=1] %tmp553554 = bitcast i16* %tmp37 to i8* ; [#uses=2] %tmp583584 = sext i16 %tmp98 to i32 ; [#uses=1] %tmp585 = sub i32 32, %tmp583584 ; [#uses=1] %tmp614615 = sext i16 %tmp101 to i32 ; [#uses=1] %tmp621622 = sext i16 %tmp104 to i32 ; [#uses=1] %tmp623 = sub i32 32, %tmp621622 ; [#uses=1] br label %bb114 produces: LBB3_5: # bb114.preheader movswl -68(%ebp), %eax movl $32, %ecx movl %ecx, -80(%ebp) subl %eax, -80(%ebp) movswl -52(%ebp), %eax movl %ecx, -84(%ebp) subl %eax, -84(%ebp) movswl -70(%ebp), %eax movl %ecx, -88(%ebp) subl %eax, -88(%ebp) movswl -50(%ebp), %eax subl %eax, %ecx movl %ecx, -76(%ebp) movswl -42(%ebp), %eax movl %eax, -92(%ebp) movswl -66(%ebp), %eax movl %eax, -96(%ebp) movw $0, -98(%ebp) This appears to be bad because the RA is not folding the store to the stack slot into the movl. The above instructions could be: movl $32, -80(%ebp) ... movl $32, -84(%ebp) ... This seems like a cross between remat and spill folding. This has redundant subtractions of %eax from a stack slot. However, %ecx doesn't change, so we could simply subtract %eax from %ecx first and then use %ecx (or vice-versa). //===---------------------------------------------------------------------===// For this code: cond_next603: ; preds = %bb493, %cond_true336, %cond_next599 %v.21050.1 = phi i32 [ %v.21050.0, %cond_next599 ], [ %tmp344, %cond_true336 ], [ %v.2, %bb493 ] ; [#uses=1] %maxz.21051.1 = phi i32 [ %maxz.21051.0, %cond_next599 ], [ 0, %cond_true336 ], [ %maxz.2, %bb493 ] ; [#uses=2] %cnt.01055.1 = phi i32 [ %cnt.01055.0, %cond_next599 ], [ 0, %cond_true336 ], [ %cnt.0, %bb493 ] ; [#uses=2] %byteptr.9 = phi i8* [ %byteptr.12, %cond_next599 ], [ %byteptr.0, %cond_true336 ], [ %byteptr.10, %bb493 ] ; [#uses=9] %bitptr.6 = phi i32 [ %tmp5571104.1, %cond_next599 ], [ %tmp4921049, %cond_true336 ], [ %bitptr.7, %bb493 ] ; [#uses=4] %source.5 = phi i32 [ %tmp602, %cond_next599 ], [ %source.0, %cond_true336 ], [ %source.6, %bb493 ] ; [#uses=7] %tmp606 = getelementptr %struct.const_tables* @tables, i32 0, i32 0, i32 %cnt.01055.1 ; [#uses=1] %tmp607 = load i8* %tmp606, align 1 ; [#uses=1] We produce this: LBB4_70: # cond_next603 movl -20(%ebp), %esi movl L_tables$non_lazy_ptr-"L4$pb"(%esi), %esi However, ICC caches this information before the loop and produces this: movl 88(%esp), %eax #481.12 //===---------------------------------------------------------------------===// This code: %tmp659 = icmp slt i16 %tmp654, 0 ; [#uses=1] br i1 %tmp659, label %cond_true662, label %cond_next715 produces this: testw %cx, %cx movswl %cx, %esi jns LBB4_109 # cond_next715 Shark tells us that using %cx in the testw instruction is sub-optimal. It suggests using the 32-bit register (which is what ICC uses). //===---------------------------------------------------------------------===// We compile this: void compare (long long foo) { if (foo < 4294967297LL) abort(); } to: compare: subl $4, %esp cmpl $0, 8(%esp) setne %al movzbw %al, %ax cmpl $1, 12(%esp) setg %cl movzbw %cl, %cx cmove %ax, %cx testb $1, %cl jne .LBB1_2 # UnifiedReturnBlock .LBB1_1: # ifthen call abort .LBB1_2: # UnifiedReturnBlock addl $4, %esp ret (also really horrible code on ppc). This is due to the expand code for 64-bit compares. GCC produces multiple branches, which is much nicer: compare: subl $12, %esp movl 20(%esp), %edx movl 16(%esp), %eax decl %edx jle .L7 .L5: addl $12, %esp ret .p2align 4,,7 .L7: jl .L4 cmpl $0, %eax .p2align 4,,8 ja .L5 .L4: .p2align 4,,9 call abort //===---------------------------------------------------------------------===// Tail call optimization improvements: Tail call optimization currently pushes all arguments on the top of the stack (their normal place for non-tail call optimized calls) that source from the callers arguments or that source from a virtual register (also possibly sourcing from callers arguments). This is done to prevent overwriting of parameters (see example below) that might be used later. example: int callee(int32, int64); int caller(int32 arg1, int32 arg2) { int64 local = arg2 * 2; return callee(arg2, (int64)local); } [arg1] [!arg2 no longer valid since we moved local onto it] [arg2] -> [(int64) [RETADDR] local ] Moving arg1 onto the stack slot of callee function would overwrite arg2 of the caller. Possible optimizations: - Analyse the actual parameters of the callee to see which would overwrite a caller parameter which is used by the callee and only push them onto the top of the stack. int callee (int32 arg1, int32 arg2); int caller (int32 arg1, int32 arg2) { return callee(arg1,arg2); } Here we don't need to write any variables to the top of the stack since they don't overwrite each other. int callee (int32 arg1, int32 arg2); int caller (int32 arg1, int32 arg2) { return callee(arg2,arg1); } Here we need to push the arguments because they overwrite each other. //===---------------------------------------------------------------------===// main () { int i = 0; unsigned long int z = 0; do { z -= 0x00004000; i++; if (i > 0x00040000) abort (); } while (z > 0); exit (0); } gcc compiles this to: _main: subl $28, %esp xorl %eax, %eax jmp L2 L3: cmpl $262144, %eax je L10 L2: addl $1, %eax cmpl $262145, %eax jne L3 call L_abort$stub L10: movl $0, (%esp) call L_exit$stub llvm: _main: subl $12, %esp movl $1, %eax movl $16384, %ecx LBB1_1: # bb cmpl $262145, %eax jge LBB1_4 # cond_true LBB1_2: # cond_next incl %eax addl $4294950912, %ecx cmpl $16384, %ecx jne LBB1_1 # bb LBB1_3: # bb11 xorl %eax, %eax addl $12, %esp ret LBB1_4: # cond_true call L_abort$stub 1. LSR should rewrite the first cmp with induction variable %ecx. 2. DAG combiner should fold leal 1(%eax), %edx cmpl $262145, %edx => cmpl $262144, %eax //===---------------------------------------------------------------------===// define i64 @test(double %X) { %Y = fptosi double %X to i64 ret i64 %Y } compiles to: _test: subl $20, %esp movsd 24(%esp), %xmm0 movsd %xmm0, 8(%esp) fldl 8(%esp) fisttpll (%esp) movl 4(%esp), %edx movl (%esp), %eax addl $20, %esp #FP_REG_KILL ret This should just fldl directly from the input stack slot. //===---------------------------------------------------------------------===// This code: int foo (int x) { return (x & 65535) | 255; } Should compile into: _foo: movzwl 4(%esp), %eax orl $255, %eax ret instead of: _foo: movl $255, %eax orl 4(%esp), %eax andl $65535, %eax ret //===---------------------------------------------------------------------===// We're codegen'ing multiply of long longs inefficiently: unsigned long long LLM(unsigned long long arg1, unsigned long long arg2) { return arg1 * arg2; } We compile to (fomit-frame-pointer): _LLM: pushl %esi movl 8(%esp), %ecx movl 16(%esp), %esi movl %esi, %eax mull %ecx imull 12(%esp), %esi addl %edx, %esi imull 20(%esp), %ecx movl %esi, %edx addl %ecx, %edx popl %esi ret This looks like a scheduling deficiency and lack of remat of the load from the argument area. ICC apparently produces: movl 8(%esp), %ecx imull 12(%esp), %ecx movl 16(%esp), %eax imull 4(%esp), %eax addl %eax, %ecx movl 4(%esp), %eax mull 12(%esp) addl %ecx, %edx ret Note that it remat'd loads from 4(esp) and 12(esp). See this GCC PR: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17236 //===---------------------------------------------------------------------===// We can fold a store into "zeroing a reg". Instead of: xorl %eax, %eax movl %eax, 124(%esp) we should get: movl $0, 124(%esp) if the flags of the xor are dead. Likewise, we isel "x<<1" into "add reg,reg". If reg is spilled, this should be folded into: shl [mem], 1 //===---------------------------------------------------------------------===// This testcase misses a read/modify/write opportunity (from PR1425): void vertical_decompose97iH1(int *b0, int *b1, int *b2, int width){ int i; for(i=0; i>0; } We compile it down to: LBB1_2: # bb movl (%esi,%edi,4), %ebx addl (%ecx,%edi,4), %ebx addl (%edx,%edi,4), %ebx movl %ebx, (%ecx,%edi,4) incl %edi cmpl %eax, %edi jne LBB1_2 # bb the inner loop should add to the memory location (%ecx,%edi,4), saving a mov. Something like: movl (%esi,%edi,4), %ebx addl (%edx,%edi,4), %ebx addl %ebx, (%ecx,%edi,4) Here is another interesting example: void vertical_compose97iH1(int *b0, int *b1, int *b2, int width){ int i; for(i=0; i>0; } We miss the r/m/w opportunity here by using 2 subs instead of an add+sub[mem]: LBB9_2: # bb movl (%ecx,%edi,4), %ebx subl (%esi,%edi,4), %ebx subl (%edx,%edi,4), %ebx movl %ebx, (%ecx,%edi,4) incl %edi cmpl %eax, %edi jne LBB9_2 # bb Additionally, LSR should rewrite the exit condition of these loops to use a stride-4 IV, would would allow all the scales in the loop to go away. This would result in smaller code and more efficient microops. //===---------------------------------------------------------------------===// In SSE mode, we turn abs and neg into a load from the constant pool plus a xor or and instruction, for example: xorpd LCPI1_0, %xmm2 However, if xmm2 gets spilled, we end up with really ugly code like this: movsd (%esp), %xmm0 xorpd LCPI1_0, %xmm0 movsd %xmm0, (%esp) Since we 'know' that this is a 'neg', we can actually "fold" the spill into the neg/abs instruction, turning it into an *integer* operation, like this: xorl 2147483648, [mem+4] ## 2147483648 = (1 << 31) you could also use xorb, but xorl is less likely to lead to a partial register stall. Here is a contrived testcase: double a, b, c; void test(double *P) { double X = *P; a = X; bar(); X = -X; b = X; bar(); c = X; } //===---------------------------------------------------------------------===// handling llvm.memory.barrier on pre SSE2 cpus should generate: lock ; mov %esp, %esp //===---------------------------------------------------------------------===// The generated code on x86 for checking for signed overflow on a multiply the obvious way is much longer than it needs to be. int x(int a, int b) { long long prod = (long long)a*b; return prod > 0x7FFFFFFF || prod < (-0x7FFFFFFF-1); } See PR2053 for more details. //===---------------------------------------------------------------------===// We should investigate using cdq/ctld (effect: edx = sar eax, 31) more aggressively; it should cost the same as a move+shift on any modern processor, but it's a lot shorter. Downside is that it puts more pressure on register allocation because it has fixed operands. Example: int abs(int x) {return x < 0 ? -x : x;} gcc compiles this to the following when using march/mtune=pentium2/3/4/m/etc.: abs: movl 4(%esp), %eax cltd xorl %edx, %eax subl %edx, %eax ret //===---------------------------------------------------------------------===// Consider: int test(unsigned long a, unsigned long b) { return -(a < b); } We currently compile this to: define i32 @test(i32 %a, i32 %b) nounwind { %tmp3 = icmp ult i32 %a, %b ; [#uses=1] %tmp34 = zext i1 %tmp3 to i32 ; [#uses=1] %tmp5 = sub i32 0, %tmp34 ; [#uses=1] ret i32 %tmp5 } and _test: movl 8(%esp), %eax cmpl %eax, 4(%esp) setb %al movzbl %al, %eax negl %eax ret Several deficiencies here. First, we should instcombine zext+neg into sext: define i32 @test2(i32 %a, i32 %b) nounwind { %tmp3 = icmp ult i32 %a, %b ; [#uses=1] %tmp34 = sext i1 %tmp3 to i32 ; [#uses=1] ret i32 %tmp34 } However, before we can do that, we have to fix the bad codegen that we get for sext from bool: _test2: movl 8(%esp), %eax cmpl %eax, 4(%esp) setb %al movzbl %al, %eax shll $31, %eax sarl $31, %eax ret This code should be at least as good as the code above. Once this is fixed, we can optimize this specific case even more to: movl 8(%esp), %eax xorl %ecx, %ecx cmpl %eax, 4(%esp) sbbl %ecx, %ecx //===---------------------------------------------------------------------===// Take the following code (from http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16541): extern unsigned char first_one[65536]; int FirstOnet(unsigned long long arg1) { if (arg1 >> 48) return (first_one[arg1 >> 48]); return 0; } The following code is currently generated: FirstOnet: movl 8(%esp), %eax cmpl $65536, %eax movl 4(%esp), %ecx jb .LBB1_2 # UnifiedReturnBlock .LBB1_1: # ifthen shrl $16, %eax movzbl first_one(%eax), %eax ret .LBB1_2: # UnifiedReturnBlock xorl %eax, %eax ret There are a few possible improvements here: 1. We should be able to eliminate the dead load into %ecx 2. We could change the "movl 8(%esp), %eax" into "movzwl 10(%esp), %eax"; this lets us change the cmpl into a testl, which is shorter, and eliminate the shift. We could also in theory eliminate the branch by using a conditional for the address of the load, but that seems unlikely to be worthwhile in general. //===---------------------------------------------------------------------===// We compile this function: define i32 @foo(i32 %a, i32 %b, i32 %c, i8 zeroext %d) nounwind { entry: %tmp2 = icmp eq i8 %d, 0 ; [#uses=1] br i1 %tmp2, label %bb7, label %bb bb: ; preds = %entry %tmp6 = add i32 %b, %a ; [#uses=1] ret i32 %tmp6 bb7: ; preds = %entry %tmp10 = sub i32 %a, %c ; [#uses=1] ret i32 %tmp10 } to: _foo: cmpb $0, 16(%esp) movl 12(%esp), %ecx movl 8(%esp), %eax movl 4(%esp), %edx je LBB1_2 # bb7 LBB1_1: # bb addl %edx, %eax ret LBB1_2: # bb7 movl %edx, %eax subl %ecx, %eax ret The coalescer could coalesce "edx" with "eax" to avoid the movl in LBB1_2 if it commuted the addl in LBB1_1. //===---------------------------------------------------------------------===// These two functions perform identical operations: define i32 @test(i32 %f12) { %tmp7.25 = lshr i32 %f12, 16 %tmp7.26 = trunc i32 %tmp7.25 to i8 %tmp78.2 = sext i8 %tmp7.26 to i32 ret i32 %tmp78.2 } define i32 @test2(i32 %f12) { %f11 = shl i32 %f12, 8 %tmp7.25 = ashr i32 %f11, 24 ret i32 %tmp7.25 } but the first compiles into significantly better code on x86-32: _test: movsbl 6(%esp), %eax ret _test2: movl 4(%esp), %eax shll $8, %eax sarl $24, %eax ret and on x86-64: _test: shrl $16, %edi movsbl %dil, %eax ret _test2: shll $8, %edi movl %edi, %eax sarl $24, %eax ret I would like instcombine to canonicalize the first into the second (since it is shorter and doesn't involve type width changes) but the x86 backend needs to do the right thing with the later sequence first. //===---------------------------------------------------------------------===//