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[InstCombine] fold popcount of exactly one bit to shift 

This is discussed in https://llvm.org/PR48999 ,
but it does not solve that request.

The difference in the vector test shows that some
other logic transform is limited to scalar types.
This commit is contained in:
Sanjay Patel 2021-04-04 11:38:09 -04:00
parent f18efb70c4
commit c0645f1324
2 changed files with 14 additions and 8 deletions
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16
llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
View File

@ -522,18 +522,24 @@ static Instruction *foldCtpop(IntrinsicInst &II, InstCombinerImpl &IC) {
return CallInst::Create(F, {X, IC.Builder.getFalse()});
}
KnownBits Known(BitWidth);
IC.computeKnownBits(Op0, Known, 0, &II);
// If all bits are zero except for exactly one fixed bit, then the result
// must be 0 or 1, and we can get that answer by shifting to LSB:
// ctpop (X & 32) --> (X & 32) >> 5
if ((~Known.Zero).isPowerOf2())
return BinaryOperator::CreateLShr(
Op0, ConstantInt::get(Ty, (~Known.Zero).exactLogBase2()));
// FIXME: Try to simplify vectors of integers.
auto *IT = dyn_cast<IntegerType>(Ty);
if (!IT)
return nullptr;
KnownBits Known(BitWidth);
IC.computeKnownBits(Op0, Known, 0, &II);
// Add range metadata since known bits can't completely reflect what we know.
unsigned MinCount = Known.countMinPopulation();
unsigned MaxCount = Known.countMaxPopulation();
// Add range metadata since known bits can't completely reflect what we know.
if (IT->getBitWidth() != 1 && !II.getMetadata(LLVMContext::MD_range)) {
Metadata *LowAndHigh[] = {
ConstantAsMetadata::get(ConstantInt::get(IT, MinCount)),

6
llvm/test/Transforms/InstCombine/ctpop.ll
View File

@ -96,8 +96,8 @@ define i1 @test6(i1 %arg) {
define i8 @mask_one_bit(i8 %x) {
; CHECK-LABEL: @mask_one_bit(
; CHECK-NEXT: [[A:%.*]] = and i8 [[X:%.*]], 16
; CHECK-NEXT: [[R:%.*]] = call i8 @llvm.ctpop.i8(i8 [[A]]), !range [[RNG1:![0-9]+]]
; CHECK-NEXT: [[A:%.*]] = lshr i8 [[X:%.*]], 4
; CHECK-NEXT: [[R:%.*]] = and i8 [[A]], 1
; CHECK-NEXT: ret i8 [[R]]
;
%a = and i8 %x, 16
@ -109,7 +109,7 @@ define <2 x i32> @mask_one_bit_splat(<2 x i32> %x, <2 x i32>* %p) {
; CHECK-LABEL: @mask_one_bit_splat(
; CHECK-NEXT: [[A:%.*]] = and <2 x i32> [[X:%.*]], <i32 2048, i32 2048>
; CHECK-NEXT: store <2 x i32> [[A]], <2 x i32>* [[P:%.*]], align 8
; CHECK-NEXT: [[R:%.*]] = call <2 x i32> @llvm.ctpop.v2i32(<2 x i32> [[A]])
; CHECK-NEXT: [[R:%.*]] = lshr exact <2 x i32> [[A]], <i32 11, i32 11>
; CHECK-NEXT: ret <2 x i32> [[R]]
;
%a = and <2 x i32> %x, <i32 2048, i32 2048>