//===---------------------------------------------------------------------===// // Random ideas for the X86 backend. //===---------------------------------------------------------------------===// Missing features: - Support for SSE4: http://www.intel.com/software/penryn http://softwarecommunity.intel.com/isn/Downloads/Intel%20SSE4%20Programming%20Reference.pdf - support for 3DNow! - weird abis? //===---------------------------------------------------------------------===// Add a MUL2U and MUL2S nodes to represent a multiply that returns both the Hi and Lo parts (combination of MUL and MULH[SU] into one node). Add this to X86, & make the dag combiner produce it when needed. This will eliminate one imul from the code generated for: long long test(long long X, long long Y) { return X*Y; } by using the EAX result from the mul. We should add a similar node for DIVREM. another case is: long long test(int X, int Y) { return (long long)X*Y; } ... which should only be one imul instruction. or: unsigned long long int t2(unsigned int a, unsigned int b) { return (unsigned long long)a * b; } ... which should be one mul instruction. This can be done with a custom expander, but it would be nice to move this to generic code. //===---------------------------------------------------------------------===// 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. 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 //===---------------------------------------------------------------------===// 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. //===---------------------------------------------------------------------===// Count leading zeros and count trailing zeros: int clz(int X) { return __builtin_clz(X); } int ctz(int X) { return __builtin_ctz(X); } $ gcc t.c -S -o - -O3 -fomit-frame-pointer -masm=intel clz: bsr %eax, DWORD PTR [%esp+4] xor %eax, 31 ret ctz: bsf %eax, DWORD PTR [%esp+4] ret however, check that these are defined for 0 and 32. Our intrinsics are, GCC's aren't. Another example (use predsimplify to eliminate a select): int foo (unsigned long j) { if (j) return __builtin_ffs (j) - 1; else return 0; } //===---------------------------------------------------------------------===// Use push/pop instructions in prolog/epilog sequences instead of stores off ESP (certain code size win, perf win on some [which?] processors). Also, 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: int %test1(int %X) { %Y = div int %X, 8 ret int %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 :) //===---------------------------------------------------------------------===// The first BB of this code: declare bool %foo() int %bar() { %V = call bool %foo() br bool %V, label %T, label %F T: ret int 1 F: call bool %foo() ret int 12 } compiles to: _bar: subl $12, %esp call L_foo$stub xorb $1, %al testb %al, %al jne LBB_bar_2 # F It would be better to emit "cmp %al, 1" than a xor and test. //===---------------------------------------------------------------------===// Enable X86InstrInfo::convertToThreeAddress(). //===---------------------------------------------------------------------===// 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$non_lazy_ptr, %edx" out of the loop after remateralization is implemented. This can be accomplished with 1) a target dependent LICM pass or 2) makeing SelectDAG represent the whole function. //===---------------------------------------------------------------------===// The following tests perform worse with LSR: lambda, siod, optimizer-eval, ackermann, hash2, nestedloop, strcat, and Treesor. //===---------------------------------------------------------------------===// We are generating far worse code than gcc: volatile short X, Y; void foo(int N) { int i; for (i = 0; i < N; i++) { X = i; Y = i*4; } } LBB1_1: #bb.preheader xorl %ecx, %ecx xorw %dx, %dx LBB1_2: #bb movl L_X$non_lazy_ptr, %esi movw %dx, (%esi) movw %dx, %si shlw $2, %si movl L_Y$non_lazy_ptr, %edi movw %si, (%edi) incl %ecx incw %dx 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 There are 3 issues: 1. Lack of post regalloc LICM. 2. Poor sub-regclass support. That leads to inability to promote the 16-bit arithmetic op to 32-bit and making use of leal. 3. LSR unable to reused IV for a different type (i16 vs. i32) even though the cast would be free. //===---------------------------------------------------------------------===// 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). //===---------------------------------------------------------------------===// Bad codegen: char foo(int x) { return x; } _foo: movl 4(%esp), %eax shll $24, %eax sarl $24, %eax ret SIGN_EXTEND_INREG can be implemented as (sext (trunc)) to take advantage of sub-registers. //===---------------------------------------------------------------------===// Consider this: typedef struct pair { float A, B; } pair; void pairtest(pair P, float *FP) { *FP = P.A+P.B; } We currently generate this code with llvmgcc4: _pairtest: movl 8(%esp), %eax movl 4(%esp), %ecx movd %eax, %xmm0 movd %ecx, %xmm1 addss %xmm0, %xmm1 movl 12(%esp), %eax movss %xmm1, (%eax) ret we should be able to generate: _pairtest: movss 4(%esp), %xmm0 movl 12(%esp), %eax addss 8(%esp), %xmm0 movss %xmm0, (%eax) ret The issue is that llvmgcc4 is forcing the struct to memory, then passing it as integer chunks. It does this so that structs like {short,short} are passed in a single 32-bit integer stack slot. We should handle the safe cases above much nicer, while still handling the hard cases. While true in general, in this specific case we could do better by promoting load int + bitcast to float -> load fload. This basically needs alignment info, the code is already implemented (but disabled) in dag combine). //===---------------------------------------------------------------------===// 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. //===---------------------------------------------------------------------===// Don't forget to find a way to squash noop truncates in the JIT environment. //===---------------------------------------------------------------------===// Implement anyext in the same manner as truncate that would allow them to be eliminated. //===---------------------------------------------------------------------===// How about implementing truncate / anyext as a property of machine instruction operand? i.e. Print as 32-bit super-class register / 16-bit sub-class register. Do this for the cases where a truncate / anyext is guaranteed to be eliminated. For IA32 that is truncate from 32 to 16 and anyext from 16 to 32. //===---------------------------------------------------------------------===// 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. //===---------------------------------------------------------------------===// Implement CTTZ, CTLZ with bsf and bsr. //===---------------------------------------------------------------------===// 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. //===---------------------------------------------------------------------===// int %foo(int* %a, int %t) { entry: br label %cond_true cond_true: ; preds = %cond_true, %entry %x.0.0 = phi int [ 0, %entry ], [ %tmp9, %cond_true ] %t_addr.0.0 = phi int [ %t, %entry ], [ %tmp7, %cond_true ] %tmp2 = getelementptr int* %a, int %x.0.0 %tmp3 = load int* %tmp2 ; [#uses=1] %tmp5 = add int %t_addr.0.0, %x.0.0 ; [#uses=1] %tmp7 = add int %tmp5, %tmp3 ; [#uses=2] %tmp9 = add int %x.0.0, 1 ; [#uses=2] %tmp = setgt int %tmp9, 39 ; [#uses=1] br bool %tmp, label %bb12, label %cond_true bb12: ; preds = %cond_true ret int %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. //===---------------------------------------------------------------------===// 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. //===---------------------------------------------------------------------===// We currently compile sign_extend_inreg into two shifts: long foo(long X) { return (long)(signed char)X; } becomes: _foo: movl 4(%esp), %eax shll $24, %eax sarl $24, %eax ret This could be: _foo: movsbl 4(%esp),%eax ret //===---------------------------------------------------------------------===// Consider the expansion of: uint %test3(uint %X) { %tmp1 = rem uint %X, 255 ret uint %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 //===---------------------------------------------------------------------===// This: #include int foo(double X) { return isnan(X); } compiles to (-m64): _foo: pxor %xmm1, %xmm1 ucomisd %xmm1, %xmm0 setp %al movzbl %al, %eax ret the pxor is not needed, we could compare the value against itself. //===---------------------------------------------------------------------===// 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. 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-upgrade < %s | llvm-as | llc -march=x86 -x86-asm-syntax=intel | \ ; RUN: not grep {mov E.X, E.X} %G = external global int int %test1(int %X, int %Y) { %Z = add int %X, %Y volatile store int %Y, int* %G volatile store int %Z, int* %G ret int %X } int %test2(int %X) { %Z = add int %X, 1 ;; inc volatile store int %Z, int* %G ret int %X } //===---------------------------------------------------------------------===// We use push/pop of stack space around calls in situations where we don't have to. Call to f below produces: subl $16, %esp <<<<< movl %eax, (%esp) call L_f$stub addl $16, %esp <<<<< The stack push/pop can be moved into the prolog/epilog. It does this because it's building the frame pointer, but this should not be sufficient, only the use of alloca should cause it to do this. (There are other issues shown by this code, but this is one.) typedef struct _range_t { float fbias; float fscale; int ibias; int iscale; int ishift; unsigned char lut[]; } range_t; struct _decode_t { int type:4; int unit:4; int alpha:8; int N:8; int bpc:8; int bpp:16; int skip:8; int swap:8; const range_t*const*range; }; typedef struct _decode_t decode_t; extern int f(const decode_t* decode); int decode_byte (const decode_t* decode) { if (decode->swap != 0) return f(decode); return 0; } //===---------------------------------------------------------------------===// This: #include unsigned test(float f) { return _mm_cvtsi128_si32( (__m128i) _mm_set_ss( f )); } Compiles to: _test: movss 4(%esp), %xmm0 movd %xmm0, %eax ret it should compile to a move from the stack slot directly into eax. DAGCombine has this xform, but it is currently disabled until the alignment fields of the load/store nodes are trustworthy. //===---------------------------------------------------------------------===// 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