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When optimizing for size, don't tail-merge unless it's likely to be a

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code-size win, and not when it's only likely to be code-size neutral,
such as when only a single instruction would be eliminated and a new
branch would be required.

This fixes rdar://7392894.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@88692 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Dan Gohman 2009-11-13 21:02:15 +00:00
parent 236490d870
commit c4c550c758
2 changed files with 125 additions and 9 deletions
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21
lib/CodeGen/BranchFolding.cpp
View File

@ -519,21 +519,24 @@ static bool ProfitableToMerge(MachineBasicBlock *MBB1,
return true;
// If both blocks have an unconditional branch temporarily stripped out,
// treat that as an additional common instruction.
if (MBB1 != PredBB && MBB2 != PredBB &&
// count that as an additional common instruction for the following
// heuristics.
unsigned EffectiveTailLen = CommonTailLen;
if (SuccBB && MBB1 != PredBB && MBB2 != PredBB &&
!MBB1->back().getDesc().isBarrier() &&
!MBB2->back().getDesc().isBarrier())
--minCommonTailLength;
++EffectiveTailLen;
// Check if the common tail is long enough to be worthwhile.
if (CommonTailLen >= minCommonTailLength)
if (EffectiveTailLen >= minCommonTailLength)
return true;
// If we are optimizing for code size, 1 instruction in common is enough if
// we don't have to split a block. At worst we will be replacing a
// fallthrough into the common tail with a branch, which at worst breaks
// even with falling through into the duplicated common tail.
if (MF->getFunction()->hasFnAttr(Attribute::OptimizeForSize) &&
// If we are optimizing for code size, 2 instructions in common is enough if
// we don't have to split a block. At worst we will be introducing 1 new
// branch instruction, which is likely to be smaller than the 2
// instructions that would be deleted in the merge.
if (EffectiveTailLen >= 2 &&
MF->getFunction()->hasFnAttr(Attribute::OptimizeForSize) &&
(I1 == MBB1->begin() || I2 == MBB2->begin()))
return true;

113
test/CodeGen/X86/tail-opts.ll
View File

@ -293,3 +293,116 @@ return:
}
declare void @func()
; one - One instruction may be tail-duplicated even with optsize.
; CHECK: one:
; CHECK: movl $0, XYZ(%rip)
; CHECK: movl $0, XYZ(%rip)
@XYZ = external global i32
define void @one() nounwind optsize {
entry:
%0 = icmp eq i32 undef, 0
br i1 %0, label %bbx, label %bby
bby:
switch i32 undef, label %bb7 [
i32 16, label %return
]
bb7:
volatile store i32 0, i32* @XYZ
unreachable
bbx:
switch i32 undef, label %bb12 [
i32 128, label %return
]
bb12:
volatile store i32 0, i32* @XYZ
unreachable
return:
ret void
}
; two - Same as one, but with two instructions in the common
; tail instead of one. This is too much to be merged, given
; the optsize attribute.
; CHECK: two:
; CHECK-NOT: XYZ
; CHECK: movl $0, XYZ(%rip)
; CHECK: movl $1, XYZ(%rip)
; CHECK-NOT: XYZ
; CHECK: ret
define void @two() nounwind optsize {
entry:
%0 = icmp eq i32 undef, 0
br i1 %0, label %bbx, label %bby
bby:
switch i32 undef, label %bb7 [
i32 16, label %return
]
bb7:
volatile store i32 0, i32* @XYZ
volatile store i32 1, i32* @XYZ
unreachable
bbx:
switch i32 undef, label %bb12 [
i32 128, label %return
]
bb12:
volatile store i32 0, i32* @XYZ
volatile store i32 1, i32* @XYZ
unreachable
return:
ret void
}
; two_nosize - Same as two, but without the optsize attribute.
; Now two instructions are enough to be tail-duplicated.
; CHECK: two_nosize:
; CHECK: movl $0, XYZ(%rip)
; CHECK: movl $1, XYZ(%rip)
; CHECK: movl $0, XYZ(%rip)
; CHECK: movl $1, XYZ(%rip)
define void @two_nosize() nounwind {
entry:
%0 = icmp eq i32 undef, 0
br i1 %0, label %bbx, label %bby
bby:
switch i32 undef, label %bb7 [
i32 16, label %return
]
bb7:
volatile store i32 0, i32* @XYZ
volatile store i32 1, i32* @XYZ
unreachable
bbx:
switch i32 undef, label %bb12 [
i32 128, label %return
]
bb12:
volatile store i32 0, i32* @XYZ
volatile store i32 1, i32* @XYZ
unreachable
return:
ret void
}