diff --git a/llvm/lib/Analysis/InstructionSimplify.cpp b/llvm/lib/Analysis/InstructionSimplify.cpp --- a/llvm/lib/Analysis/InstructionSimplify.cpp +++ b/llvm/lib/Analysis/InstructionSimplify.cpp @@ -1445,10 +1445,11 @@ simplifyShift(Instruction::Shl, Op0, Op1, IsNSW, Q, MaxRecurse)) return V; + Type * Ty = Op0->getType(); // undef << X -> 0 // undef << X -> undef if (if it's NSW/NUW) if (Q.isUndefValue(Op0)) - return IsNSW || IsNUW ? Op0 : Constant::getNullValue(Op0->getType()); + return IsNSW || IsNUW ? Op0 : Constant::getNullValue(Ty); // (X >> A) << A -> X Value *X; @@ -1462,6 +1463,9 @@ // NOTE: could use computeKnownBits() / LazyValueInfo, // but the cost-benefit analysis suggests it isn't worth it. + if (IsNSW && IsNUW && match(Op1, m_SpecificInt(Ty->getScalarSizeInBits() - 1))) + return Constant::getNullValue(Ty); + return nullptr; } diff --git a/llvm/test/Transforms/InstSimplify/shift.ll b/llvm/test/Transforms/InstSimplify/shift.ll --- a/llvm/test/Transforms/InstSimplify/shift.ll +++ b/llvm/test/Transforms/InstSimplify/shift.ll @@ -350,3 +350,51 @@ %vc = lshr %va, shufflevector ( insertelement ( poison, i16 16, i32 0), poison, zeroinitializer) ret %vc } + +; shl nsw+nuw is 0 +define i8 @shl_nsw_nuw_7_eq_0(i8 %x) { +; CHECK-LABEL: @shl_nsw_nuw_7_eq_0( +; CHECK-NEXT: ret i8 0 +; + %y = shl nsw nuw i8 %x, 7 + ret i8 %y +} + +; Make sure we are match element width +define <2 x i8> @shl_vec_nsw_nuw_7_eq_0(<2 x i8> %x) { +; CHECK-LABEL: @shl_vec_nsw_nuw_7_eq_0( +; CHECK-NEXT: ret <2 x i8> zeroinitializer +; + %y = shl nsw nuw <2 x i8> %x, + ret <2 x i8> %y +} + +; negative test (missing nuw) +define i8 @shl_nsw_7_fail_missing_nuw(i8 %x) { +; CHECK-LABEL: @shl_nsw_7_fail_missing_nuw( +; CHECK-NEXT: [[Y:%.*]] = shl nsw i8 [[X:%.*]], 7 +; CHECK-NEXT: ret i8 [[Y]] +; + %y = shl nsw i8 %x, 7 + ret i8 %y +} + +; negative test (missing nsw) +define i8 @shl_nuw_7_fail_missing_nsw(i8 %x) { +; CHECK-LABEL: @shl_nuw_7_fail_missing_nsw( +; CHECK-NEXT: [[Y:%.*]] = shl nuw i8 [[X:%.*]], 7 +; CHECK-NEXT: ret i8 [[Y]] +; + %y = shl nuw i8 %x, 7 + ret i8 %y +} + +; negative test (shift value != bitwdth - 1) +define i8 @shl_nsw_nuw_6_do_nothing(i8 %x) { +; CHECK-LABEL: @shl_nsw_nuw_6_do_nothing( +; CHECK-NEXT: [[Y:%.*]] = shl nuw nsw i8 [[X:%.*]], 6 +; CHECK-NEXT: ret i8 [[Y]] +; + %y = shl nsw nuw i8 %x, 6 + ret i8 %y +}