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When optimizing for size, don't tail-merge unless it's likely to be a
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
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@ -519,21 +519,24 @@ static bool ProfitableToMerge(MachineBasicBlock *MBB1,
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return true;
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// If both blocks have an unconditional branch temporarily stripped out,
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// treat that as an additional common instruction.
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if (MBB1 != PredBB && MBB2 != PredBB &&
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// count that as an additional common instruction for the following
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// heuristics.
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unsigned EffectiveTailLen = CommonTailLen;
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if (SuccBB && MBB1 != PredBB && MBB2 != PredBB &&
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!MBB1->back().getDesc().isBarrier() &&
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!MBB2->back().getDesc().isBarrier())
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--minCommonTailLength;
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++EffectiveTailLen;
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// Check if the common tail is long enough to be worthwhile.
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if (CommonTailLen >= minCommonTailLength)
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if (EffectiveTailLen >= minCommonTailLength)
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return true;
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// If we are optimizing for code size, 1 instruction in common is enough if
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// we don't have to split a block. At worst we will be replacing a
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// fallthrough into the common tail with a branch, which at worst breaks
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// even with falling through into the duplicated common tail.
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if (MF->getFunction()->hasFnAttr(Attribute::OptimizeForSize) &&
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// If we are optimizing for code size, 2 instructions in common is enough if
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// we don't have to split a block. At worst we will be introducing 1 new
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// branch instruction, which is likely to be smaller than the 2
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// instructions that would be deleted in the merge.
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if (EffectiveTailLen >= 2 &&
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MF->getFunction()->hasFnAttr(Attribute::OptimizeForSize) &&
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(I1 == MBB1->begin() || I2 == MBB2->begin()))
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return true;
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@ -293,3 +293,116 @@ return:
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}
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declare void @func()
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; one - One instruction may be tail-duplicated even with optsize.
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; CHECK: one:
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; CHECK: movl $0, XYZ(%rip)
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; CHECK: movl $0, XYZ(%rip)
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@XYZ = external global i32
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define void @one() nounwind optsize {
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entry:
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%0 = icmp eq i32 undef, 0
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br i1 %0, label %bbx, label %bby
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bby:
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switch i32 undef, label %bb7 [
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i32 16, label %return
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]
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bb7:
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volatile store i32 0, i32* @XYZ
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unreachable
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bbx:
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switch i32 undef, label %bb12 [
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i32 128, label %return
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]
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bb12:
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volatile store i32 0, i32* @XYZ
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unreachable
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return:
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ret void
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}
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; two - Same as one, but with two instructions in the common
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; tail instead of one. This is too much to be merged, given
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; the optsize attribute.
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; CHECK: two:
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; CHECK-NOT: XYZ
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; CHECK: movl $0, XYZ(%rip)
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; CHECK: movl $1, XYZ(%rip)
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; CHECK-NOT: XYZ
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; CHECK: ret
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define void @two() nounwind optsize {
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entry:
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%0 = icmp eq i32 undef, 0
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br i1 %0, label %bbx, label %bby
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bby:
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switch i32 undef, label %bb7 [
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i32 16, label %return
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]
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bb7:
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volatile store i32 0, i32* @XYZ
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volatile store i32 1, i32* @XYZ
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unreachable
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bbx:
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switch i32 undef, label %bb12 [
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i32 128, label %return
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]
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bb12:
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volatile store i32 0, i32* @XYZ
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volatile store i32 1, i32* @XYZ
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unreachable
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return:
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ret void
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}
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; two_nosize - Same as two, but without the optsize attribute.
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; Now two instructions are enough to be tail-duplicated.
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; CHECK: two_nosize:
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; CHECK: movl $0, XYZ(%rip)
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; CHECK: movl $1, XYZ(%rip)
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; CHECK: movl $0, XYZ(%rip)
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; CHECK: movl $1, XYZ(%rip)
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define void @two_nosize() nounwind {
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entry:
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%0 = icmp eq i32 undef, 0
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br i1 %0, label %bbx, label %bby
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bby:
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switch i32 undef, label %bb7 [
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i32 16, label %return
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]
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bb7:
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volatile store i32 0, i32* @XYZ
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volatile store i32 1, i32* @XYZ
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unreachable
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bbx:
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switch i32 undef, label %bb12 [
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i32 128, label %return
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]
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bb12:
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volatile store i32 0, i32* @XYZ
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volatile store i32 1, i32* @XYZ
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unreachable
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return:
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ret void
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}
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