// Random ideas for the ARM backend.
//===---------------------------------------------------------------------===//
-Consider implementing a select with two conditional moves:
+Reimplement 'select' in terms of 'SEL'.
-cmp x, y
-moveq dst, a
-movne dst, b
+* We would really like to support UXTAB16, but we need to prove that the
+ add doesn't need to overflow between the two 16-bit chunks.
-----------------------------------------------------------
+* implement predication support
+* Implement pre/post increment support. (e.g. PR935)
+* Coalesce stack slots!
+* Implement smarter constant generation for binops with large immediates.
+* Consider materializing FP constants like 0.0f and 1.0f using integer
+ immediate instructions then copy to FPU. Slower than load into FPU?
-%tmp1 = shl int %b, ubyte %c
-%tmp4 = add int %a, %tmp1
+//===---------------------------------------------------------------------===//
+
+Crazy idea: Consider code that uses lots of 8-bit or 16-bit values. By the
+time regalloc happens, these values are now in a 32-bit register, usually with
+the top-bits known to be sign or zero extended. If spilled, we should be able
+to spill these to a 8-bit or 16-bit stack slot, zero or sign extending as part
+of the reload.
+
+Doing this reduces the size of the stack frame (important for thumb etc), and
+also increases the likelihood that we will be able to reload multiple values
+from the stack with a single load.
+
+//===---------------------------------------------------------------------===//
+
+The constant island pass is in good shape. Some cleanups might be desirable,
+but there is unlikely to be much improvement in the generated code.
+
+1. There may be some advantage to trying to be smarter about the initial
+placement, rather than putting everything at the end.
+
+2. The handling of 2-byte padding for Thumb is overly conservative. There
+would be a small gain to keeping accurate track of the padding (which would
+require aligning functions containing constant pools to 4-byte boundaries).
+
+3. There might be some compile-time efficiency to be had by representing
+consecutive islands as a single block rather than multiple blocks.
+
+4. Use a priority queue to sort constant pool users in inverse order of
+ position so we always process the one closed to the end of functions
+ first. This may simply CreateNewWater.
+
+//===---------------------------------------------------------------------===//
+
+We need to start generating predicated instructions. The .td files have a way
+to express this now (see the PPC conditional return instruction), but the
+branch folding pass (or a new if-cvt pass) should start producing these, at
+least in the trivial case.
+
+Among the obvious wins, doing so can eliminate the need to custom expand
+copysign (i.e. we won't need to custom expand it to get the conditional
+negate).
+
+This allows us to eliminate one instruction from:
+
+define i32 @_Z6slow4bii(i32 %x, i32 %y) {
+ %tmp = icmp sgt i32 %x, %y
+ %retval = select i1 %tmp, i32 %x, i32 %y
+ ret i32 %retval
+}
+
+__Z6slow4bii:
+ cmp r0, r1
+ movgt r1, r0
+ mov r0, r1
+ bx lr
+
+//===---------------------------------------------------------------------===//
+
+Implement long long "X-3" with instructions that fold the immediate in. These
+were disabled due to badness with the ARM carry flag on subtracts.
+
+//===---------------------------------------------------------------------===//
+
+We currently compile abs:
+int foo(int p) { return p < 0 ? -p : p; }
+
+into:
+
+_foo:
+ rsb r1, r0, #0
+ cmn r0, #1
+ movgt r1, r0
+ mov r0, r1
+ bx lr
+
+This is very, uh, literal. This could be a 3 operation sequence:
+ t = (p sra 31);
+ res = (p xor t)-t
+
+Which would be better. This occurs in png decode.
+
+//===---------------------------------------------------------------------===//
+
+More load / store optimizations:
+1) Look past instructions without side-effects (not load, store, branch, etc.)
+ when forming the list of loads / stores to optimize.
+
+2) Smarter register allocation?
+We are probably missing some opportunities to use ldm / stm. Consider:
+
+ldr r5, [r0]
+ldr r4, [r0, #4]
+
+This cannot be merged into a ldm. Perhaps we will need to do the transformation
+before register allocation. Then teach the register allocator to allocate a
+chunk of consecutive registers.
+
+3) Better representation for block transfer? This is from Olden/power:
-compiles to
+ fldd d0, [r4]
+ fstd d0, [r4, #+32]
+ fldd d0, [r4, #+8]
+ fstd d0, [r4, #+40]
+ fldd d0, [r4, #+16]
+ fstd d0, [r4, #+48]
+ fldd d0, [r4, #+24]
+ fstd d0, [r4, #+56]
-add r0, r0, r1, lsl r2
+If we can spare the registers, it would be better to use fldm and fstm here.
+Need major register allocator enhancement though.
-but
+4) Can we recognize the relative position of constantpool entries? i.e. Treat
-%tmp1 = shl int %b, ubyte %c
-%tmp4 = add int %tmp1, %a
+ ldr r0, LCPI17_3
+ ldr r1, LCPI17_4
+ ldr r2, LCPI17_5
-compiles to
-mov r1, r1, lsl r2
-add r0, r1, r0
+ as
+ ldr r0, LCPI17
+ ldr r1, LCPI17+4
+ ldr r2, LCPI17+8
-----------------------------------------------------------
+ Then the ldr's can be combined into a single ldm. See Olden/power.
-add an offset to FLDS/FLDD addressing mode
+Note for ARM v4 gcc uses ldmia to load a pair of 32-bit values to represent a
+double 64-bit FP constant:
-----------------------------------------------------------
+ adr r0, L6
+ ldmia r0, {r0-r1}
-the function
+ .align 2
+L6:
+ .long -858993459
+ .long 1074318540
-void %f() {
+5) Can we make use of ldrd and strd? Instead of generating ldm / stm, use
+ldrd/strd instead if there are only two destination registers that form an
+odd/even pair. However, we probably would pay a penalty if the address is not
+aligned on 8-byte boundary. This requires more information on load / store
+nodes (and MI's?) then we currently carry.
+
+6) struct copies appear to be done field by field
+instead of by words, at least sometimes:
+
+struct foo { int x; short s; char c1; char c2; };
+void cpy(struct foo*a, struct foo*b) { *a = *b; }
+
+llvm code (-O2)
+ ldrb r3, [r1, #+6]
+ ldr r2, [r1]
+ ldrb r12, [r1, #+7]
+ ldrh r1, [r1, #+4]
+ str r2, [r0]
+ strh r1, [r0, #+4]
+ strb r3, [r0, #+6]
+ strb r12, [r0, #+7]
+gcc code (-O2)
+ ldmia r1, {r1-r2}
+ stmia r0, {r1-r2}
+
+In this benchmark poor handling of aggregate copies has shown up as
+having a large effect on size, and possibly speed as well (we don't have
+a good way to measure on ARM).
+
+//===---------------------------------------------------------------------===//
+
+* Consider this silly example:
+
+double bar(double x) {
+ double r = foo(3.1);
+ return x+r;
+}
+
+_bar:
+ sub sp, sp, #16
+ str r4, [sp, #+12]
+ str r5, [sp, #+8]
+ str lr, [sp, #+4]
+ mov r4, r0
+ mov r5, r1
+ ldr r0, LCPI2_0
+ bl _foo
+ fmsr f0, r0
+ fcvtsd d0, f0
+ fmdrr d1, r4, r5
+ faddd d0, d0, d1
+ fmrrd r0, r1, d0
+ ldr lr, [sp, #+4]
+ ldr r5, [sp, #+8]
+ ldr r4, [sp, #+12]
+ add sp, sp, #16
+ bx lr
+
+Ignore the prologue and epilogue stuff for a second. Note
+ mov r4, r0
+ mov r5, r1
+the copys to callee-save registers and the fact they are only being used by the
+fmdrr instruction. It would have been better had the fmdrr been scheduled
+before the call and place the result in a callee-save DPR register. The two
+mov ops would not have been necessary.
+
+//===---------------------------------------------------------------------===//
+
+Calling convention related stuff:
+
+* gcc's parameter passing implementation is terrible and we suffer as a result:
+
+e.g.
+struct s {
+ double d1;
+ int s1;
+};
+
+void foo(struct s S) {
+ printf("%g, %d\n", S.d1, S.s1);
+}
+
+'S' is passed via registers r0, r1, r2. But gcc stores them to the stack, and
+then reload them to r1, r2, and r3 before issuing the call (r0 contains the
+address of the format string):
+
+ stmfd sp!, {r7, lr}
+ add r7, sp, #0
+ sub sp, sp, #12
+ stmia sp, {r0, r1, r2}
+ ldmia sp, {r1-r2}
+ ldr r0, L5
+ ldr r3, [sp, #8]
+L2:
+ add r0, pc, r0
+ bl L_printf$stub
+
+Instead of a stmia, ldmia, and a ldr, wouldn't it be better to do three moves?
+
+* Return an aggregate type is even worse:
+
+e.g.
+struct s foo(void) {
+ struct s S = {1.1, 2};
+ return S;
+}
+
+ mov ip, r0
+ ldr r0, L5
+ sub sp, sp, #12
+L2:
+ add r0, pc, r0
+ @ lr needed for prologue
+ ldmia r0, {r0, r1, r2}
+ stmia sp, {r0, r1, r2}
+ stmia ip, {r0, r1, r2}
+ mov r0, ip
+ add sp, sp, #12
+ bx lr
+
+r0 (and later ip) is the hidden parameter from caller to store the value in. The
+first ldmia loads the constants into r0, r1, r2. The last stmia stores r0, r1,
+r2 into the address passed in. However, there is one additional stmia that
+stores r0, r1, and r2 to some stack location. The store is dead.
+
+The llvm-gcc generated code looks like this:
+
+csretcc void %foo(%struct.s* %agg.result) {
entry:
- call void %g( int 1, int 2, int 3, int 4, int 5 )
+ %S = alloca %struct.s, align 4 ; <%struct.s*> [#uses=1]
+ %memtmp = alloca %struct.s ; <%struct.s*> [#uses=1]
+ cast %struct.s* %S to sbyte* ; <sbyte*>:0 [#uses=2]
+ call void %llvm.memcpy.i32( sbyte* %0, sbyte* cast ({ double, int }* %C.0.904 to sbyte*), uint 12, uint 4 )
+ cast %struct.s* %agg.result to sbyte* ; <sbyte*>:1 [#uses=2]
+ call void %llvm.memcpy.i32( sbyte* %1, sbyte* %0, uint 12, uint 0 )
+ cast %struct.s* %memtmp to sbyte* ; <sbyte*>:2 [#uses=1]
+ call void %llvm.memcpy.i32( sbyte* %2, sbyte* %1, uint 12, uint 0 )
ret void
}
-declare void %g(int, int, int, int, int)
+llc ends up issuing two memcpy's (the first memcpy becomes 3 loads from
+constantpool). Perhaps we should 1) fix llvm-gcc so the memcpy is translated
+into a number of load and stores, or 2) custom lower memcpy (of small size) to
+be ldmia / stmia. I think option 2 is better but the current register
+allocator cannot allocate a chunk of registers at a time.
+
+A feasible temporary solution is to use specific physical registers at the
+lowering time for small (<= 4 words?) transfer size.
+
+* ARM CSRet calling convention requires the hidden argument to be returned by
+the callee.
+
+//===---------------------------------------------------------------------===//
+
+We can definitely do a better job on BB placements to eliminate some branches.
+It's very common to see llvm generated assembly code that looks like this:
+
+LBB3:
+ ...
+LBB4:
+...
+ beq LBB3
+ b LBB2
+
+If BB4 is the only predecessor of BB3, then we can emit BB3 after BB4. We can
+then eliminate beq and and turn the unconditional branch to LBB2 to a bne.
+
+See McCat/18-imp/ComputeBoundingBoxes for an example.
+
+//===---------------------------------------------------------------------===//
+
+Register scavenging is now implemented. The example in the previous version
+of this document produces optimal code at -O2.
+
+//===---------------------------------------------------------------------===//
+
+Pre-/post- indexed load / stores:
+
+1) We should not make the pre/post- indexed load/store transform if the base ptr
+is guaranteed to be live beyond the load/store. This can happen if the base
+ptr is live out of the block we are performing the optimization. e.g.
+
+mov r1, r2
+ldr r3, [r1], #4
+...
+
+vs.
+
+ldr r3, [r2]
+add r1, r2, #4
+...
+
+In most cases, this is just a wasted optimization. However, sometimes it can
+negatively impact the performance because two-address code is more restrictive
+when it comes to scheduling.
+
+Unfortunately, liveout information is currently unavailable during DAG combine
+time.
+
+2) Consider spliting a indexed load / store into a pair of add/sub + load/store
+ to solve #1 (in TwoAddressInstructionPass.cpp).
+
+3) Enhance LSR to generate more opportunities for indexed ops.
+
+4) Once we added support for multiple result patterns, write indexed loads
+ patterns instead of C++ instruction selection code.
+
+5) Use FLDM / FSTM to emulate indexed FP load / store.
+
+//===---------------------------------------------------------------------===//
+
+We should add i64 support to take advantage of the 64-bit load / stores.
+We can add a pseudo i64 register class containing pseudo registers that are
+register pairs. All other ops (e.g. add, sub) would be expanded as usual.
+
+We need to add pseudo instructions (i.e. gethi / getlo) to extract i32 registers
+from the i64 register. These are single moves which can be eliminated if the
+destination register is a sub-register of the source. We should implement proper
+subreg support in the register allocator to coalesce these away.
+
+There are other minor issues such as multiple instructions for a spill / restore
+/ move.
+
+//===---------------------------------------------------------------------===//
+
+Implement support for some more tricky ways to materialize immediates. For
+example, to get 0xffff8000, we can use:
+
+mov r9, #&3f8000
+sub r9, r9, #&400000
+
+//===---------------------------------------------------------------------===//
+
+We sometimes generate multiple add / sub instructions to update sp in prologue
+and epilogue if the inc / dec value is too large to fit in a single immediate
+operand. In some cases, perhaps it might be better to load the value from a
+constantpool instead.
+
+//===---------------------------------------------------------------------===//
+
+GCC generates significantly better code for this function.
+
+int foo(int StackPtr, unsigned char *Line, unsigned char *Stack, int LineLen) {
+ int i = 0;
+
+ if (StackPtr != 0) {
+ while (StackPtr != 0 && i < (((LineLen) < (32768))? (LineLen) : (32768)))
+ Line[i++] = Stack[--StackPtr];
+ if (LineLen > 32768)
+ {
+ while (StackPtr != 0 && i < LineLen)
+ {
+ i++;
+ --StackPtr;
+ }
+ }
+ }
+ return StackPtr;
+}
+
+//===---------------------------------------------------------------------===//
+
+This should compile to the mlas instruction:
+int mlas(int x, int y, int z) { return ((x * y + z) < 0) ? 7 : 13; }
+
+//===---------------------------------------------------------------------===//
+
+At some point, we should triage these to see if they still apply to us:
+
+http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19598
+http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18560
+http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27016
+
+http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11831
+http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11826
+http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11825
+http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11824
+http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11823
+http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11820
+http://gcc.gnu.org/bugzilla/show_bug.cgi?id=10982
+
+http://gcc.gnu.org/bugzilla/show_bug.cgi?id=10242
+http://gcc.gnu.org/bugzilla/show_bug.cgi?id=9831
+http://gcc.gnu.org/bugzilla/show_bug.cgi?id=9760
+http://gcc.gnu.org/bugzilla/show_bug.cgi?id=9759
+http://gcc.gnu.org/bugzilla/show_bug.cgi?id=9703
+http://gcc.gnu.org/bugzilla/show_bug.cgi?id=9702
+http://gcc.gnu.org/bugzilla/show_bug.cgi?id=9663
+
+http://www.inf.u-szeged.hu/gcc-arm/
+http://citeseer.ist.psu.edu/debus04linktime.html
+
+//===---------------------------------------------------------------------===//
+
+gcc generates smaller code for this function at -O2 or -Os:
+
+void foo(signed char* p) {
+ if (*p == 3)
+ bar();
+ else if (*p == 4)
+ baz();
+ else if (*p == 5)
+ quux();
+}
+
+llvm decides it's a good idea to turn the repeated if...else into a
+binary tree, as if it were a switch; the resulting code requires -1
+compare-and-branches when *p<=2 or *p==5, the same number if *p==4
+or *p>6, and +1 if *p==3. So it should be a speed win
+(on balance). However, the revised code is larger, with 4 conditional
+branches instead of 3.
-Only needs 8 bytes of stack space. We currently allocate 16.
+More seriously, there is a byte->word extend before
+each comparison, where there should be only one, and the condition codes
+are not remembered when the same two values are compared twice.
-----------------------------------------------------------
+//===---------------------------------------------------------------------===//
+
+More register scavenging work:
+
+1. Use the register scavenger to track frame index materialized into registers
+ (those that do not fit in addressing modes) to allow reuse in the same BB.
+2. Finish scavenging for Thumb.
+3. We know some spills and restores are unnecessary. The issue is once live
+ intervals are merged, they are not never split. So every def is spilled
+ and every use requires a restore if the register allocator decides the
+ resulting live interval is not assigned a physical register. It may be
+ possible (with the help of the scavenger) to turn some spill / restore
+ pairs into register copies.
+
+//===---------------------------------------------------------------------===//
-32 x 32 -> 64 multiplications currently uses two instructions. We
-should try to declare smull and umull as returning two values.
+More LSR enhancements possible:
-----------------------------------------------------------
+1. Teach LSR about pre- and post- indexed ops to allow iv increment be merged
+ in a load / store.
+2. Allow iv reuse even when a type conversion is required. For example, i8
+ and i32 load / store addressing modes are identical.
-Implement addressing modes 2 (ldrb) and 3 (ldrsb)
-----------------------------------------------------------
+//===---------------------------------------------------------------------===//
+
+This:
+
+int foo(int a, int b, int c, int d) {
+ long long acc = (long long)a * (long long)b;
+ acc += (long long)c * (long long)d;
+ return (int)(acc >> 32);
+}
+
+Should compile to use SMLAL (Signed Multiply Accumulate Long) which multiplies
+two signed 32-bit values to produce a 64-bit value, and accumulates this with
+a 64-bit value.
+
+We currently get this with v6:
+
+_foo:
+ mul r12, r1, r0
+ smmul r1, r1, r0
+ smmul r0, r3, r2
+ mul r3, r3, r2
+ adds r3, r3, r12
+ adc r0, r0, r1
+ bx lr
+
+and this with v4:
+
+_foo:
+ stmfd sp!, {r7, lr}
+ mov r7, sp
+ mul r12, r1, r0
+ smull r0, r1, r1, r0
+ smull lr, r0, r3, r2
+ mul r3, r3, r2
+ adds r3, r3, r12
+ adc r0, r0, r1
+ ldmfd sp!, {r7, pc}
+
+This apparently occurs in real code.
+
+//===---------------------------------------------------------------------===//
projects / oota-llvm.git / blobdiff