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[InstCombine] Combine trunc (lshr X, BW-1) to i1 --> icmp slt X, 0 (#142593) (#143846)
Fixes #142593, the issue was fixed using the suggestion on the ticket itself. Godbolt: https://godbolt.org/z/oW5b74jc4 alive2 proof: https://alive2.llvm.org/ce/z/QHnD7e
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‎llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp

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Original file line numberDiff line numberDiff line change
@@ -815,6 +815,12 @@ Instruction *InstCombinerImpl::visitTrunc(TruncInst &Trunc) {
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return new ICmpInst(ICmpInst::ICMP_EQ, X, CmpC);
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}
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if (match(Src, m_Shr(m_Value(X), m_SpecificInt(SrcWidth - 1)))) {
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// trunc (ashr X, BW-1) to i1 --> icmp slt X, 0
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// trunc (lshr X, BW-1) to i1 --> icmp slt X, 0
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return new ICmpInst(ICmpInst::ICMP_SLT, X, Zero);
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}
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Constant *C;
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if (match(Src, m_OneUse(m_LShr(m_Value(X), m_ImmConstant(C))))) {
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// trunc (lshr X, C) to i1 --> icmp ne (and X, C'), 0

‎llvm/test/Transforms/InstCombine/logical-select.ll

Lines changed: 2 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -807,9 +807,9 @@ define <2 x i16> @bitcast_vec_cond_commute3(<4 x i8> %cond, <2 x i16> %pc, <2 x
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; CHECK-LABEL: @bitcast_vec_cond_commute3(
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; CHECK-NEXT: [[C:%.*]] = mul <2 x i16> [[PC:%.*]], [[PC]]
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; CHECK-NEXT: [[D:%.*]] = mul <2 x i16> [[PD:%.*]], [[PD]]
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; CHECK-NEXT: [[DOTNOT2:%.*]] = icmp slt <4 x i8> [[COND:%.*]], zeroinitializer
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; CHECK-NEXT: [[TMP1:%.*]] = bitcast <2 x i16> [[D]] to <4 x i8>
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; CHECK-NEXT: [[TMP2:%.*]] = bitcast <2 x i16> [[C]] to <4 x i8>
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; CHECK-NEXT: [[DOTNOT2:%.*]] = icmp slt <4 x i8> [[COND:%.*]], zeroinitializer
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; CHECK-NEXT: [[TMP3:%.*]] = select <4 x i1> [[DOTNOT2]], <4 x i8> [[TMP1]], <4 x i8> [[TMP2]]
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; CHECK-NEXT: [[R:%.*]] = bitcast <4 x i8> [[TMP3]] to <2 x i16>
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; CHECK-NEXT: ret <2 x i16> [[R]]
@@ -1069,8 +1069,8 @@ define <2 x i1> @not_d_bools_vector_poison(<2 x i1> %c, <2 x i1> %x, <2 x i1> %y
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define i32 @not_d_allSignBits(i32 %cond, i32 %tval, i32 %fval) {
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; CHECK-LABEL: @not_d_allSignBits(
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; CHECK-NEXT: [[TMP1:%.*]] = xor i32 [[FVAL:%.*]], -1
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; CHECK-NEXT: [[DOTNOT2:%.*]] = icmp slt i32 [[COND:%.*]], 0
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; CHECK-NEXT: [[TMP1:%.*]] = xor i32 [[FVAL:%.*]], -1
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; CHECK-NEXT: [[SEL:%.*]] = select i1 [[DOTNOT2]], i32 [[TVAL:%.*]], i32 [[TMP1]]
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; CHECK-NEXT: ret i32 [[SEL]]
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;

‎llvm/test/Transforms/InstCombine/trunc-lshr.ll

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Original file line numberDiff line numberDiff line change
@@ -0,0 +1,95 @@
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; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
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; RUN: opt -S -passes=instcombine < %s | FileCheck %s
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define i1 @test1(i32 %i, ptr %p) {
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; CHECK-LABEL: define i1 @test1(
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; CHECK-SAME: i32 [[I:%.*]], ptr [[P:%.*]]) {
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; CHECK-NEXT: [[DOTLOBIT:%.*]] = lshr i32 [[I]], 31
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; CHECK-NEXT: store i32 [[DOTLOBIT]], ptr [[P]], align 1
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; CHECK-NEXT: ret i1 false
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;
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%lobit = lshr i32 %i, 31
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%t = trunc nuw i32 %lobit to i1
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%b = icmp slt i32 %i, 0
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%not = xor i1 %t, true
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%op = select i1 %not, i1 %b, i1 false
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store i32 %lobit, ptr %p, align 1
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ret i1 %op
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}
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define i1 @test2(i32 %i, ptr %p) {
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; CHECK-LABEL: define i1 @test2(
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; CHECK-SAME: i32 [[I:%.*]], ptr [[P:%.*]]) {
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; CHECK-NEXT: [[DOTLOBIT:%.*]] = ashr i32 [[I]], 31
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; CHECK-NEXT: store i32 [[DOTLOBIT]], ptr [[P]], align 1
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; CHECK-NEXT: ret i1 false
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;
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%lobit = ashr i32 %i, 31
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%t = trunc nuw i32 %lobit to i1
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%b = icmp slt i32 %i, 0
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%not = xor i1 %t, true
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%op = select i1 %not, i1 %b, i1 false
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store i32 %lobit, ptr %p, align 1
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ret i1 %op
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}
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define i1 @test3(i32 %i, ptr %p, ptr %q) {
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; CHECK-LABEL: define i1 @test3(
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; CHECK-SAME: i32 [[I:%.*]], ptr [[P:%.*]], ptr [[Q:%.*]]) {
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; CHECK-NEXT: [[DOTLOBIT:%.*]] = lshr i32 [[I]], 31
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; CHECK-NEXT: store i32 [[DOTLOBIT]], ptr [[P]], align 1
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; CHECK-NEXT: store i32 [[DOTLOBIT]], ptr [[Q]], align 1
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; CHECK-NEXT: ret i1 false
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;
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%lobit = lshr i32 %i, 31
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%t = trunc nuw i32 %lobit to i1
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%b = icmp slt i32 %i, 0
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%not = xor i1 %t, true
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%op = select i1 %not, i1 %b, i1 false
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store i32 %lobit, ptr %p, align 1
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store i32 %lobit, ptr %q, align 1
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ret i1 %op
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}
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; Negative Test
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define i1 @test4(i32 %i, ptr %p) {
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; CHECK-LABEL: define i1 @test4(
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; CHECK-SAME: i32 [[I:%.*]], ptr [[P:%.*]]) {
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; CHECK-NEXT: [[DOTLOBIT:%.*]] = lshr i32 [[I]], 30
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; CHECK-NEXT: [[T:%.*]] = trunc nuw i32 [[DOTLOBIT]] to i1
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; CHECK-NEXT: [[B:%.*]] = icmp slt i32 [[I]], 0
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; CHECK-NEXT: [[NOT_:%.*]] = xor i1 [[T]], true
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; CHECK-NEXT: [[COMMON_RET1_OP:%.*]] = select i1 [[NOT_]], i1 [[B]], i1 false
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; CHECK-NEXT: store i32 [[DOTLOBIT]], ptr [[P]], align 1
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; CHECK-NEXT: ret i1 [[COMMON_RET1_OP]]
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;
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%lobit = lshr i32 %i, 30 ; should not fold as no. of bits shifted < BitWidth - 1
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%t = trunc nuw i32 %lobit to i1
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%b = icmp slt i32 %i, 0
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%not = xor i1 %t, true
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%op = select i1 %not, i1 %b, i1 false
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store i32 %lobit, ptr %p, align 1
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ret i1 %op
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}
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; Negative Test
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define i1 @test5(i32 %i, ptr %p) {
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; CHECK-LABEL: define i1 @test5(
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; CHECK-SAME: i32 [[I:%.*]], ptr [[P:%.*]]) {
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; CHECK-NEXT: [[DOTLOBIT:%.*]] = ashr i32 [[I]], 30
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; CHECK-NEXT: [[T:%.*]] = trunc nuw i32 [[DOTLOBIT]] to i1
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; CHECK-NEXT: [[B:%.*]] = icmp slt i32 [[I]], 0
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; CHECK-NEXT: [[NOT_:%.*]] = xor i1 [[T]], true
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; CHECK-NEXT: [[COMMON_RET1_OP:%.*]] = select i1 [[NOT_]], i1 [[B]], i1 false
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; CHECK-NEXT: store i32 [[DOTLOBIT]], ptr [[P]], align 1
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; CHECK-NEXT: ret i1 [[COMMON_RET1_OP]]
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;
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%lobit = ashr i32 %i, 30 ; should not fold as no. of bits shifted < BitWidth - 1
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%t = trunc nuw i32 %lobit to i1
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%b = icmp slt i32 %i, 0
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%not = xor i1 %t, true
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%op = select i1 %not, i1 %b, i1 false
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store i32 %lobit, ptr %p, align 1
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ret i1 %op
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}
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