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[SimplifyCFG] ReduceSwitchRange: Improve on the case where the SubThreshold doesn't trigger 

llvm-svn: 361728
This commit is contained in:
Shawn Landden 2019-05-26 13:55:52 +00:00
parent 30111c786f
commit fa91ab85d9
2 changed files with 28 additions and 18 deletions
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38
llvm/lib/Transforms/Utils/SimplifyCFG.cpp
View File

@ -5552,18 +5552,6 @@ static bool ReduceSwitchRange(SwitchInst *SI, IRBuilder<> &Builder,
BestDistance = Values[I] - Values[I - 1];
}
}
uint64_t Base = 0;
// Now transform the values such that they start at zero and ascend.
if (Values[BestIndex] >= SubThreshold) {
Base = Values[BestIndex];
MadeChanges = true;
for (auto &V : Values)
V = (APInt(BitWidth, V) - Base).getLimitedValue();
}
// Now we have signed numbers that have been shifted so that, given enough
// precision, there are no negative values. Since the rest of the transform
// is bitwise only, we switch now to an unsigned representation.
// This transform can be done speculatively because it is so cheap - it
// results in a single rotate operation being inserted.
@ -5575,8 +5563,10 @@ static bool ReduceSwitchRange(SwitchInst *SI, IRBuilder<> &Builder,
// one element and LLVM disallows duplicate cases, Shift is guaranteed to be
// less than 64.
unsigned Shift = 64;
// We need to store this from _before_ the transform
uint64_t BestIndexXor = Values[BestIndex];
for (auto &V : Values)
Shift = std::min(Shift, countTrailingZeros(V));
Shift = std::min(Shift, countTrailingZeros(V ^ BestIndexXor));
assert(Shift < 64);
if (Shift > 0) {
MadeChanges = true;
@ -5584,6 +5574,26 @@ static bool ReduceSwitchRange(SwitchInst *SI, IRBuilder<> &Builder,
V >>= Shift;
}
// We Xor against Values[] (any element will do) because the if we do not
// start at zero, but also don't meet the SubThreshold, then we still might
// share common rights bits, and if this transform succeeds
// then we should insert the subtraction anyways, because the rotate trick
// below to avoid a branch needs the shifted away bits to be zero.
// Now transform the values such that they start at zero and ascend. Do not
// do this if the shift reduces the lowest value to less than SubThreshold,
// or if the subtraction is less than SubThreshold and it does not enable a
// rotate.
uint64_t Base = 0;
if ((BestIndexXor >= SubThreshold && Shift == 0) ||
(Shift > countTrailingZeros(BestIndexXor) &&
Values[BestIndex] >= SubThreshold)) {
Base = BestIndexXor;
MadeChanges = true;
for (auto &V : Values)
V = (APInt(BitWidth, V) - Base).getLimitedValue();
}
if (!MadeChanges)
// We didn't do anything.
return false;
@ -5614,7 +5624,7 @@ static bool ReduceSwitchRange(SwitchInst *SI, IRBuilder<> &Builder,
for (auto Case : SI->cases()) {
auto *Orig = Case.getCaseValue();
auto Sub = Orig->getValue() - APInt(Ty->getBitWidth(), Base);
auto Sub = Orig->getValue() - Base;
Case.setValue(cast<ConstantInt>(ConstantInt::get(Ty, Sub.lshr(Shift))));
}
return true;

8
llvm/test/Transforms/SimplifyCFG/rangereduce.ll
View File

@ -234,10 +234,10 @@ three:
define i8 @test7(i8 %a) optsize {
; CHECK-LABEL: @test7(
; CHECK-NEXT: [[TMP1:%.*]] = sub i8 [[A:%.*]], -36
; CHECK-NEXT: [[TMP2:%.*]] = lshr i8 [[TMP1]], 2
; CHECK-NEXT: [[TMP3:%.*]] = shl i8 [[TMP1]], 6
; CHECK-NEXT: [[TMP4:%.*]] = or i8 [[TMP2]], [[TMP3]]
; CHECK-NEXT: [[TMP1:%.*]] = lshr i8 [[A:%.*]], 2
; CHECK-NEXT: [[TMP2:%.*]] = shl i8 [[A]], 6
; CHECK-NEXT: [[TMP3:%.*]] = or i8 [[TMP1]], [[TMP2]]
; CHECK-NEXT: [[TMP4:%.*]] = sub i8 [[TMP3]], 55
; CHECK-NEXT: [[TMP5:%.*]] = icmp ult i8 [[TMP4]], 4
; CHECK-NEXT: br i1 [[TMP5]], label [[SWITCH_LOOKUP:%.*]], label [[DEF:%.*]]
; CHECK: switch.lookup: