[llvm] [InstCombine] Preserve NSW flags for neg instructions (PR #72548)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: Yingwei Zheng (dtcxzyw)

<details>
<summary>Changes</summary>

Alive2: https://alive2.llvm.org/ce/z/F9HG3M

This missed optimization is discovered with the help of https://github.com/AliveToolkit/alive2/pull/962.

---
Full diff: https://github.com/llvm/llvm-project/pull/72548.diff


3 Files Affected:

- (modified) llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp (+2-2) 
- (modified) llvm/test/Transforms/InstCombine/div.ll (+8-8) 
- (modified) llvm/test/Transforms/InstCombine/sdiv-exact-by-negative-power-of-two.ll (+3-3) 


``````````diff
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp b/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
index d6fac88a39d01be..40156726c7038b9 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
@@ -1433,7 +1433,7 @@ Instruction *InstCombinerImpl::visitSDiv(BinaryOperator &I) {
   // sdiv Op0, (sext i1 X) --> -Op0 (because if X is 0, the op is undefined)
   if (match(Op1, m_AllOnes()) ||
       (match(Op1, m_SExt(m_Value(X))) && X->getType()->isIntOrIntVectorTy(1)))
-    return BinaryOperator::CreateNeg(Op0);
+    return BinaryOperator::CreateNSWNeg(Op0);
 
   // X / INT_MIN --> X == INT_MIN
   if (match(Op1, m_SignMask()))
@@ -1456,7 +1456,7 @@ Instruction *InstCombinerImpl::visitSDiv(BinaryOperator &I) {
       Constant *NegPow2C = ConstantExpr::getNeg(cast<Constant>(Op1));
       Constant *C = ConstantExpr::getExactLogBase2(NegPow2C);
       Value *Ashr = Builder.CreateAShr(Op0, C, I.getName() + ".neg", true);
-      return BinaryOperator::CreateNeg(Ashr);
+      return BinaryOperator::CreateNSWNeg(Ashr);
     }
   }
 
diff --git a/llvm/test/Transforms/InstCombine/div.ll b/llvm/test/Transforms/InstCombine/div.ll
index 844b35723b135c9..cd17d10d0e1de07 100644
--- a/llvm/test/Transforms/InstCombine/div.ll
+++ b/llvm/test/Transforms/InstCombine/div.ll
@@ -22,7 +22,7 @@ define i32 @test2(i32 %A) {
 
 define i32 @sdiv_by_minus1(i32 %A) {
 ; CHECK-LABEL: @sdiv_by_minus1(
-; CHECK-NEXT:    [[B:%.*]] = sub i32 0, [[A:%.*]]
+; CHECK-NEXT:    [[B:%.*]] = sub nsw i32 0, [[A:%.*]]
 ; CHECK-NEXT:    ret i32 [[B]]
 ;
   %B = sdiv i32 %A, -1
@@ -31,7 +31,7 @@ define i32 @sdiv_by_minus1(i32 %A) {
 
 define <2 x i64> @sdiv_by_minus1_vec(<2 x i64> %x) {
 ; CHECK-LABEL: @sdiv_by_minus1_vec(
-; CHECK-NEXT:    [[DIV:%.*]] = sub <2 x i64> zeroinitializer, [[X:%.*]]
+; CHECK-NEXT:    [[DIV:%.*]] = sub nsw <2 x i64> zeroinitializer, [[X:%.*]]
 ; CHECK-NEXT:    ret <2 x i64> [[DIV]]
 ;
   %div = sdiv <2 x i64> %x, <i64 -1, i64 -1>
@@ -48,7 +48,7 @@ define <2 x i64> @sdiv_by_minus1_vec_poison_elt(<2 x i64> %x) {
 
 define i32 @sdiv_by_sext_minus1(i1 %x, i32 %y) {
 ; CHECK-LABEL: @sdiv_by_sext_minus1(
-; CHECK-NEXT:    [[DIV:%.*]] = sub i32 0, [[Y:%.*]]
+; CHECK-NEXT:    [[DIV:%.*]] = sub nsw i32 0, [[Y:%.*]]
 ; CHECK-NEXT:    ret i32 [[DIV]]
 ;
   %sext = sext i1 %x to i32
@@ -58,7 +58,7 @@ define i32 @sdiv_by_sext_minus1(i1 %x, i32 %y) {
 
 define <2 x i32> @sdiv_by_sext_minus1_vec(<2 x i1> %x, <2 x i32> %y) {
 ; CHECK-LABEL: @sdiv_by_sext_minus1_vec(
-; CHECK-NEXT:    [[DIV:%.*]] = sub <2 x i32> zeroinitializer, [[Y:%.*]]
+; CHECK-NEXT:    [[DIV:%.*]] = sub nsw <2 x i32> zeroinitializer, [[Y:%.*]]
 ; CHECK-NEXT:    ret <2 x i32> [[DIV]]
 ;
   %sext = sext <2 x i1> %x to <2 x i32>
@@ -1308,8 +1308,8 @@ define i32 @udiv_select_of_constants_divisor(i1 %b, i32 %x) {
 define i1 @sdiv_one_icmpeq_one(i32 %x) {
 ; CHECK-LABEL: @sdiv_one_icmpeq_one(
 ; CHECK-NEXT:    [[X_FR:%.*]] = freeze i32 [[X:%.*]]
-; CHECK-NEXT:    [[B:%.*]] = icmp eq i32 [[X_FR]], 1
-; CHECK-NEXT:    ret i1 [[B]]
+; CHECK-NEXT:    [[B1:%.*]] = icmp eq i32 [[X_FR]], 1
+; CHECK-NEXT:    ret i1 [[B1]]
 ;
   %A = sdiv i32 1, %x
   %B = icmp eq i32 %A, 1
@@ -1319,8 +1319,8 @@ define i1 @sdiv_one_icmpeq_one(i32 %x) {
 define i1 @sdiv_one_icmpeq_negone(i32 %x) {
 ; CHECK-LABEL: @sdiv_one_icmpeq_negone(
 ; CHECK-NEXT:    [[X_FR:%.*]] = freeze i32 [[X:%.*]]
-; CHECK-NEXT:    [[B:%.*]] = icmp eq i32 [[X_FR]], -1
-; CHECK-NEXT:    ret i1 [[B]]
+; CHECK-NEXT:    [[B1:%.*]] = icmp eq i32 [[X_FR]], -1
+; CHECK-NEXT:    ret i1 [[B1]]
 ;
   %A = sdiv i32 1, %x
   %B = icmp eq i32 %A, -1
diff --git a/llvm/test/Transforms/InstCombine/sdiv-exact-by-negative-power-of-two.ll b/llvm/test/Transforms/InstCombine/sdiv-exact-by-negative-power-of-two.ll
index 81f22bb3c6646f7..3451c5e626f308b 100644
--- a/llvm/test/Transforms/InstCombine/sdiv-exact-by-negative-power-of-two.ll
+++ b/llvm/test/Transforms/InstCombine/sdiv-exact-by-negative-power-of-two.ll
@@ -38,7 +38,7 @@ define <2 x i8> @t2_vec_splat(<2 x i8> %x) {
 define <2 x i8> @t3_vec(<2 x i8> %x) {
 ; CHECK-LABEL: @t3_vec(
 ; CHECK-NEXT:    [[DIV_NEG:%.*]] = ashr exact <2 x i8> [[X:%.*]], <i8 5, i8 4>
-; CHECK-NEXT:    [[DIV:%.*]] = sub <2 x i8> zeroinitializer, [[DIV_NEG]]
+; CHECK-NEXT:    [[DIV:%.*]] = sub nsw <2 x i8> zeroinitializer, [[DIV_NEG]]
 ; CHECK-NEXT:    ret <2 x i8> [[DIV]]
 ;
   %div = sdiv exact <2 x i8> %x, <i8 -32, i8 -16>
@@ -76,8 +76,8 @@ define i8 @prove_exact_with_high_mask(i8 %x, i8 %y) {
 
 define i8 @prove_exact_with_high_mask_limit(i8 %x, i8 %y) {
 ; CHECK-LABEL: @prove_exact_with_high_mask_limit(
-; CHECK-NEXT:    [[A:%.*]] = ashr i8 [[X:%.*]], 5
-; CHECK-NEXT:    [[D:%.*]] = sub nsw i8 0, [[A]]
+; CHECK-NEXT:    [[D_NEG:%.*]] = ashr i8 [[X:%.*]], 5
+; CHECK-NEXT:    [[D:%.*]] = sub nsw i8 0, [[D_NEG]]
 ; CHECK-NEXT:    ret i8 [[D]]
 ;
   %a = and i8 %x, -32

``````````

</details>


https://github.com/llvm/llvm-project/pull/72548