From 3a126134d38144928de802bf96d45e72dd1b26d2 Mon Sep 17 00:00:00 2001 From: Sanjay Patel Date: Sun, 12 Sep 2021 09:18:14 -0400 Subject: [PATCH] [InstCombine] remove casts from splat-a-bit pattern https://alive2.llvm.org/ce/z/_AivbM This case seems clear since we can reduce instruction count and avoid an intermediate type change, but we might want to use mask-and-compare for other sequences. Currently, we can generate more instructions on some related patterns by trying to use bit-hacks instead of mask+cmp, so something is not behaving as expected. --- llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp | 12 ++++++++++++ llvm/test/Transforms/InstCombine/sext.ll | 17 ++++++++++++----- 2 files changed, 24 insertions(+), 5 deletions(-) diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp index bb9120d..8a918a0 100644 --- a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp +++ b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp @@ -1591,6 +1591,18 @@ Instruction *InstCombinerImpl::visitSExt(SExtInst &CI) { return BinaryOperator::CreateAShr(A, NewShAmt); } + // Splatting a bit of constant-index across a value: + // sext (ashr (trunc iN X to iM), M-1) to iN --> ashr (shl X, N-M), N-1 + // TODO: If the dest type is different, use a cast (adjust use check). + if (match(Src, m_OneUse(m_AShr(m_Trunc(m_Value(X)), + m_SpecificInt(SrcBitSize - 1)))) && + X->getType() == DestTy) { + Constant *ShlAmtC = ConstantInt::get(DestTy, DestBitSize - SrcBitSize); + Constant *AshrAmtC = ConstantInt::get(DestTy, DestBitSize - 1); + Value *Shl = Builder.CreateShl(X, ShlAmtC); + return BinaryOperator::CreateAShr(Shl, AshrAmtC); + } + if (match(Src, m_VScale(DL))) { if (CI.getFunction()->hasFnAttribute(Attribute::VScaleRange)) { unsigned MaxVScale = CI.getFunction() diff --git a/llvm/test/Transforms/InstCombine/sext.ll b/llvm/test/Transforms/InstCombine/sext.ll index eec2f9b..1240b69 100644 --- a/llvm/test/Transforms/InstCombine/sext.ll +++ b/llvm/test/Transforms/InstCombine/sext.ll @@ -323,9 +323,8 @@ define i10 @test19(i10 %i) { define i32 @smear_set_bit(i32 %x) { ; CHECK-LABEL: @smear_set_bit( -; CHECK-NEXT: [[T:%.*]] = trunc i32 [[X:%.*]] to i8 -; CHECK-NEXT: [[A:%.*]] = ashr i8 [[T]], 7 -; CHECK-NEXT: [[S:%.*]] = sext i8 [[A]] to i32 +; CHECK-NEXT: [[TMP1:%.*]] = shl i32 [[X:%.*]], 24 +; CHECK-NEXT: [[S:%.*]] = ashr i32 [[TMP1]], 31 ; CHECK-NEXT: ret i32 [[S]] ; %t = trunc i32 %x to i8 @@ -334,12 +333,14 @@ define i32 @smear_set_bit(i32 %x) { ret i32 %s } +; extra use of trunc is ok because we still shorten the use chain + define <2 x i32> @smear_set_bit_vec_use1(<2 x i32> %x) { ; CHECK-LABEL: @smear_set_bit_vec_use1( ; CHECK-NEXT: [[T:%.*]] = trunc <2 x i32> [[X:%.*]] to <2 x i5> ; CHECK-NEXT: call void @use_vec(<2 x i5> [[T]]) -; CHECK-NEXT: [[A:%.*]] = ashr <2 x i5> [[T]], -; CHECK-NEXT: [[S:%.*]] = sext <2 x i5> [[A]] to <2 x i32> +; CHECK-NEXT: [[TMP1:%.*]] = shl <2 x i32> [[X]], +; CHECK-NEXT: [[S:%.*]] = ashr <2 x i32> [[TMP1]], ; CHECK-NEXT: ret <2 x i32> [[S]] ; %t = trunc <2 x i32> %x to <2 x i5> @@ -349,6 +350,8 @@ define <2 x i32> @smear_set_bit_vec_use1(<2 x i32> %x) { ret <2 x i32> %s } +; negative test - extra use + define i32 @smear_set_bit_use2(i32 %x) { ; CHECK-LABEL: @smear_set_bit_use2( ; CHECK-NEXT: [[T:%.*]] = trunc i32 [[X:%.*]] to i8 @@ -364,6 +367,8 @@ define i32 @smear_set_bit_use2(i32 %x) { ret i32 %s } +; negative test - must shift all the way across + define i32 @smear_set_bit_wrong_shift_amount(i32 %x) { ; CHECK-LABEL: @smear_set_bit_wrong_shift_amount( ; CHECK-NEXT: [[T:%.*]] = trunc i32 [[X:%.*]] to i8 @@ -377,6 +382,8 @@ define i32 @smear_set_bit_wrong_shift_amount(i32 %x) { ret i32 %s } +; TODO: this could be mask+compare+sext or shifts+trunc + define i16 @smear_set_bit_different_dest_type(i32 %x) { ; CHECK-LABEL: @smear_set_bit_different_dest_type( ; CHECK-NEXT: [[T:%.*]] = trunc i32 [[X:%.*]] to i8 -- 2.7.4