From af2b0bef4361e8308697022f55209199190f08a3 Mon Sep 17 00:00:00 2001 From: Nikita Popov Date: Thu, 28 Feb 2019 18:04:20 +0000 Subject: [PATCH] [ValueTracking] More accurate unsigned sub overflow detection Second part of D58593. Compute precise overflow conditions based on all known bits, rather than just the sign bits. Unsigned a - b overflows iff a < b, and we can determine whether this always/never happens based on the minimal and maximal values achievable for a and b subject to the known bits constraint. llvm-svn: 355109 --- llvm/lib/Analysis/ValueTracking.cpp | 20 +++++++++---------- .../InstCombine/demand_shrink_nsw.ll | 2 +- .../InstCombine/saturating-add-sub.ll | 12 ++++------- llvm/test/Transforms/InstCombine/sext.ll | 2 +- llvm/test/Transforms/InstCombine/strlen-1.ll | 2 +- llvm/test/Transforms/InstCombine/sub-xor.ll | 2 +- llvm/test/Transforms/InstCombine/wcslen-1.ll | 6 +++--- llvm/test/Transforms/InstCombine/wcslen-3.ll | 6 +++--- .../LoopVectorize/X86/masked_load_store.ll | 2 +- 9 files changed, 24 insertions(+), 30 deletions(-) diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp index f6e3886f652e..8b3d16a51257 100644 --- a/llvm/lib/Analysis/ValueTracking.cpp +++ b/llvm/lib/Analysis/ValueTracking.cpp @@ -4165,18 +4165,16 @@ OverflowResult llvm::computeOverflowForUnsignedSub(const Value *LHS, const Instruction *CxtI, const DominatorTree *DT) { KnownBits LHSKnown = computeKnownBits(LHS, DL, /*Depth=*/0, AC, CxtI, DT); - if (LHSKnown.isNonNegative() || LHSKnown.isNegative()) { - KnownBits RHSKnown = computeKnownBits(RHS, DL, /*Depth=*/0, AC, CxtI, DT); - - // If the LHS is negative and the RHS is non-negative, no unsigned wrap. - if (LHSKnown.isNegative() && RHSKnown.isNonNegative()) - return OverflowResult::NeverOverflows; - - // If the LHS is non-negative and the RHS negative, we always wrap. - if (LHSKnown.isNonNegative() && RHSKnown.isNegative()) - return OverflowResult::AlwaysOverflows; - } + KnownBits RHSKnown = computeKnownBits(RHS, DL, /*Depth=*/0, AC, CxtI, DT); + // a - b overflows iff a < b. Determine whether this is never/always true + // based on the min/max values achievable under the known bits constraint. + APInt MinLHS = LHSKnown.One, MaxLHS = ~LHSKnown.Zero; + APInt MinRHS = RHSKnown.One, MaxRHS = ~RHSKnown.Zero; + if (MinLHS.uge(MaxRHS)) + return OverflowResult::NeverOverflows; + if (MaxLHS.ult(MinRHS)) + return OverflowResult::AlwaysOverflows; return OverflowResult::MayOverflow; } diff --git a/llvm/test/Transforms/InstCombine/demand_shrink_nsw.ll b/llvm/test/Transforms/InstCombine/demand_shrink_nsw.ll index ec007775a3d7..be19fbc4f910 100644 --- a/llvm/test/Transforms/InstCombine/demand_shrink_nsw.ll +++ b/llvm/test/Transforms/InstCombine/demand_shrink_nsw.ll @@ -11,7 +11,7 @@ define i32 @foo(i32 %arg) { ; CHECK-NEXT: [[V35:%.*]] = add nuw nsw i32 [[V34]], 1362915575 ; CHECK-NEXT: [[V40:%.*]] = shl nuw nsw i32 [[V34]], 1 ; CHECK-NEXT: [[V41:%.*]] = and i32 [[V40]], 290 -; CHECK-NEXT: [[V42:%.*]] = sub nsw i32 [[V35]], [[V41]] +; CHECK-NEXT: [[V42:%.*]] = sub nuw nsw i32 [[V35]], [[V41]] ; CHECK-NEXT: [[V43:%.*]] = add nuw i32 [[V42]], 1533579450 ; CHECK-NEXT: [[V45:%.*]] = xor i32 [[V43]], 749011377 ; CHECK-NEXT: ret i32 [[V45]] diff --git a/llvm/test/Transforms/InstCombine/saturating-add-sub.ll b/llvm/test/Transforms/InstCombine/saturating-add-sub.ll index aa833f7c728c..3afee2157012 100644 --- a/llvm/test/Transforms/InstCombine/saturating-add-sub.ll +++ b/llvm/test/Transforms/InstCombine/saturating-add-sub.ll @@ -611,7 +611,7 @@ define <2 x i8> @test_vector_usub_nneg_nneg(<2 x i8> %a) { define i8 @test_scalar_usub_never_overflows(i8 %a) { ; CHECK-LABEL: @test_scalar_usub_never_overflows( ; CHECK-NEXT: [[A_MASKED:%.*]] = or i8 [[A:%.*]], 64 -; CHECK-NEXT: [[R:%.*]] = call i8 @llvm.usub.sat.i8(i8 [[A_MASKED]], i8 10) +; CHECK-NEXT: [[R:%.*]] = add nsw i8 [[A_MASKED]], -10 ; CHECK-NEXT: ret i8 [[R]] ; %a_masked = or i8 %a, 64 @@ -622,7 +622,7 @@ define i8 @test_scalar_usub_never_overflows(i8 %a) { define <2 x i8> @test_vector_usub_never_overflows(<2 x i8> %a) { ; CHECK-LABEL: @test_vector_usub_never_overflows( ; CHECK-NEXT: [[A_MASKED:%.*]] = or <2 x i8> [[A:%.*]], -; CHECK-NEXT: [[R:%.*]] = call <2 x i8> @llvm.usub.sat.v2i8(<2 x i8> [[A_MASKED]], <2 x i8> ) +; CHECK-NEXT: [[R:%.*]] = add nsw <2 x i8> [[A_MASKED]], ; CHECK-NEXT: ret <2 x i8> [[R]] ; %a_masked = or <2 x i8> %a, @@ -632,9 +632,7 @@ define <2 x i8> @test_vector_usub_never_overflows(<2 x i8> %a) { define i8 @test_scalar_usub_always_overflows(i8 %a) { ; CHECK-LABEL: @test_scalar_usub_always_overflows( -; CHECK-NEXT: [[A_MASKED:%.*]] = and i8 [[A:%.*]], 64 -; CHECK-NEXT: [[R:%.*]] = call i8 @llvm.usub.sat.i8(i8 [[A_MASKED]], i8 100) -; CHECK-NEXT: ret i8 [[R]] +; CHECK-NEXT: ret i8 0 ; %a_masked = and i8 %a, 64 %r = call i8 @llvm.usub.sat.i8(i8 %a_masked, i8 100) @@ -643,9 +641,7 @@ define i8 @test_scalar_usub_always_overflows(i8 %a) { define <2 x i8> @test_vector_usub_always_overflows(<2 x i8> %a) { ; CHECK-LABEL: @test_vector_usub_always_overflows( -; CHECK-NEXT: [[A_MASKED:%.*]] = and <2 x i8> [[A:%.*]], -; CHECK-NEXT: [[R:%.*]] = call <2 x i8> @llvm.usub.sat.v2i8(<2 x i8> [[A_MASKED]], <2 x i8> ) -; CHECK-NEXT: ret <2 x i8> [[R]] +; CHECK-NEXT: ret <2 x i8> zeroinitializer ; %a_masked = and <2 x i8> %a, %r = call <2 x i8> @llvm.usub.sat.v2i8(<2 x i8> %a_masked, <2 x i8> ) diff --git a/llvm/test/Transforms/InstCombine/sext.ll b/llvm/test/Transforms/InstCombine/sext.ll index 46406ac2f788..faf33719927d 100644 --- a/llvm/test/Transforms/InstCombine/sext.ll +++ b/llvm/test/Transforms/InstCombine/sext.ll @@ -78,7 +78,7 @@ define i64 @test6(i32 %x) { define i64 @test7(i32 %x) { ; CHECK-LABEL: @test7( ; CHECK-NEXT: [[T:%.*]] = and i32 %x, 511 -; CHECK-NEXT: [[U:%.*]] = sub nsw i32 20000, [[T]] +; CHECK-NEXT: [[U:%.*]] = sub nuw nsw i32 20000, [[T]] ; CHECK-NEXT: [[S1:%.*]] = zext i32 [[U]] to i64 ; CHECK-NEXT: ret i64 [[S1]] ; diff --git a/llvm/test/Transforms/InstCombine/strlen-1.ll b/llvm/test/Transforms/InstCombine/strlen-1.ll index 5d8c4e028b57..aaf1b89945a0 100644 --- a/llvm/test/Transforms/InstCombine/strlen-1.ll +++ b/llvm/test/Transforms/InstCombine/strlen-1.ll @@ -128,7 +128,7 @@ define i32 @test_simplify10(i32 %x) { define i32 @test_simplify11(i32 %x) { ; CHECK-LABEL: @test_simplify11( ; CHECK-NEXT: [[AND:%.*]] = and i32 %x, 7 -; CHECK-NEXT: [[TMP1:%.*]] = sub nsw i32 9, [[AND]] +; CHECK-NEXT: [[TMP1:%.*]] = sub nuw nsw i32 9, [[AND]] ; CHECK-NEXT: ret i32 [[TMP1]] ; %and = and i32 %x, 7 diff --git a/llvm/test/Transforms/InstCombine/sub-xor.ll b/llvm/test/Transforms/InstCombine/sub-xor.ll index adcca8480594..dda5c7485406 100644 --- a/llvm/test/Transforms/InstCombine/sub-xor.ll +++ b/llvm/test/Transforms/InstCombine/sub-xor.ll @@ -39,7 +39,7 @@ define i32 @test2(i32 %x) nounwind { define i32 @test3(i32 %x) { ; CHECK-LABEL: @test3( ; CHECK-NEXT: [[AND:%.*]] = and i32 %x, 31 -; CHECK-NEXT: [[ADD:%.*]] = sub nsw i32 73, [[AND]] +; CHECK-NEXT: [[ADD:%.*]] = sub nuw nsw i32 73, [[AND]] ; CHECK-NEXT: ret i32 [[ADD]] ; %and = and i32 %x, 31 diff --git a/llvm/test/Transforms/InstCombine/wcslen-1.ll b/llvm/test/Transforms/InstCombine/wcslen-1.ll index f8f8ec12dbb4..1139048c7067 100644 --- a/llvm/test/Transforms/InstCombine/wcslen-1.ll +++ b/llvm/test/Transforms/InstCombine/wcslen-1.ll @@ -130,9 +130,9 @@ define i64 @test_simplify10(i32 %x) { define i64 @test_simplify11(i32 %x) { ; CHECK-LABEL: @test_simplify11( ; CHECK-NEXT: [[AND:%.*]] = and i32 [[X:%.*]], 7 -; CHECK-NEXT: [[TMP1:%.*]] = zext i32 [[AND]] to i64 -; CHECK-NEXT: [[TMP2:%.*]] = sub nsw i64 9, [[TMP1]] -; CHECK-NEXT: ret i64 [[TMP2]] +; CHECK-NEXT: [[NARROW:%.*]] = sub nuw nsw i32 9, [[AND]] +; CHECK-NEXT: [[TMP1:%.*]] = zext i32 [[NARROW]] to i64 +; CHECK-NEXT: ret i64 [[TMP1]] ; %and = and i32 %x, 7 %hello_p = getelementptr inbounds [13 x i32], [13 x i32]* @null_hello_mid, i32 0, i32 %and diff --git a/llvm/test/Transforms/InstCombine/wcslen-3.ll b/llvm/test/Transforms/InstCombine/wcslen-3.ll index 18382c41c83a..e789442f4905 100644 --- a/llvm/test/Transforms/InstCombine/wcslen-3.ll +++ b/llvm/test/Transforms/InstCombine/wcslen-3.ll @@ -131,9 +131,9 @@ define i64 @test_simplify10(i16 %x) { define i64 @test_simplify11(i16 %x) { ; CHECK-LABEL: @test_simplify11( ; CHECK-NEXT: [[AND:%.*]] = and i16 [[X:%.*]], 7 -; CHECK-NEXT: [[TMP1:%.*]] = zext i16 [[AND]] to i64 -; CHECK-NEXT: [[TMP2:%.*]] = sub nsw i64 9, [[TMP1]] -; CHECK-NEXT: ret i64 [[TMP2]] +; CHECK-NEXT: [[NARROW:%.*]] = sub nuw nsw i16 9, [[AND]] +; CHECK-NEXT: [[TMP1:%.*]] = zext i16 [[NARROW]] to i64 +; CHECK-NEXT: ret i64 [[TMP1]] ; %and = and i16 %x, 7 %hello_p = getelementptr inbounds [13 x i16], [13 x i16]* @null_hello_mid, i16 0, i16 %and diff --git a/llvm/test/Transforms/LoopVectorize/X86/masked_load_store.ll b/llvm/test/Transforms/LoopVectorize/X86/masked_load_store.ll index cdb985952a79..419cedbe145b 100644 --- a/llvm/test/Transforms/LoopVectorize/X86/masked_load_store.ll +++ b/llvm/test/Transforms/LoopVectorize/X86/masked_load_store.ll @@ -2028,7 +2028,7 @@ define void @foo4(double* %A, double* %B, i32* %trigger) { ; AVX512-NEXT: br i1 [[TMP23]], label [[FOR_BODY_PREHEADER]], label [[VECTOR_BODY]], !llvm.loop !49 ; AVX512: for.body.preheader: ; AVX512-NEXT: [[INDVARS_IV_PH:%.*]] = phi i64 [ 0, [[ENTRY]] ], [ 9984, [[VECTOR_BODY]] ] -; AVX512-NEXT: [[TMP24:%.*]] = sub nsw i64 9999, [[INDVARS_IV_PH]] +; AVX512-NEXT: [[TMP24:%.*]] = sub nuw nsw i64 9999, [[INDVARS_IV_PH]] ; AVX512-NEXT: br label [[FOR_BODY_PROL:%.*]] ; AVX512: for.body.prol: ; AVX512-NEXT: [[INDVARS_IV_PROL:%.*]] = phi i64 [ [[INDVARS_IV_NEXT_PROL:%.*]], [[FOR_INC_PROL:%.*]] ], [ [[INDVARS_IV_PH]], [[FOR_BODY_PREHEADER]] ]