[llvm] 8e516d4 - [InstCombine] Infer nuw flags for `C-(X+C2)` -> `(C-C2)-X` (#72373)

[via llvm-commits]

Author: Yingwei Zheng
Date: 2023-11-16T02:35:47+08:00
New Revision: 8e516d48fedc1d0b537fe0e134259ac077e5df16

URL: https://github.com/llvm/llvm-project/commit/8e516d48fedc1d0b537fe0e134259ac077e5df16
DIFF: https://github.com/llvm/llvm-project/commit/8e516d48fedc1d0b537fe0e134259ac077e5df16.diff

LOG: [InstCombine] Infer nuw flags for `C-(X+C2)` -> `(C-C2)-X` (#72373)

This patch improves https://reviews.llvm.org/D152068 by inferring NUW
flags for sub insts.
It is worth noting that we don't need to check overflow for `C-C2`.
Alive2: https://alive2.llvm.org/ce/z/uutGpS

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

Added: 
    

Modified: 
    llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
    llvm/test/Transforms/InstCombine/addsub-constant-folding.ll

Removed: 
    


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diff  --git a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
index 2bd738c95ecaa7f..318992b55e4f9f8 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -2111,14 +2111,16 @@ Instruction *InstCombinerImpl::visitSub(BinaryOperator &I) {
 
     // C-(X+C2) --> (C-C2)-X
     if (match(Op1, m_Add(m_Value(X), m_ImmConstant(C2)))) {
-      // C-C2 never overflow, and C-(X+C2), (X+C2) has NSW
-      // => (C-C2)-X can have NSW
+      // C-C2 never overflow, and C-(X+C2), (X+C2) has NSW/NUW
+      // => (C-C2)-X can have NSW/NUW
       bool WillNotSOV = willNotOverflowSignedSub(C, C2, I);
       BinaryOperator *Res =
           BinaryOperator::CreateSub(ConstantExpr::getSub(C, C2), X);
       auto *OBO1 = cast<OverflowingBinaryOperator>(Op1);
       Res->setHasNoSignedWrap(I.hasNoSignedWrap() && OBO1->hasNoSignedWrap() &&
                               WillNotSOV);
+      Res->setHasNoUnsignedWrap(I.hasNoUnsignedWrap() &&
+                                OBO1->hasNoUnsignedWrap());
       return Res;
     }
   }

diff  --git a/llvm/test/Transforms/InstCombine/addsub-constant-folding.ll b/llvm/test/Transforms/InstCombine/addsub-constant-folding.ll
index 6a6bf74d6152fa4..da92ca11a9e0d0c 100644
--- a/llvm/test/Transforms/InstCombine/addsub-constant-folding.ll
+++ b/llvm/test/Transforms/InstCombine/addsub-constant-folding.ll
@@ -175,7 +175,7 @@ define i8 @add_nsw_const_const_sub_nsw_ov(i8 %arg) {
 
 define i8 @add_nuw_const_const_sub_nuw(i8 %arg) {
 ; CHECK-LABEL: @add_nuw_const_const_sub_nuw(
-; CHECK-NEXT:    [[T1:%.*]] = sub i8 -128, [[ARG:%.*]]
+; CHECK-NEXT:    [[T1:%.*]] = sub nuw i8 -128, [[ARG:%.*]]
 ; CHECK-NEXT:    ret i8 [[T1]]
 ;
   %t0 = add nuw i8 %arg, 1
@@ -183,6 +183,16 @@ define i8 @add_nuw_const_const_sub_nuw(i8 %arg) {
   ret i8 %t1
 }
 
+define i8 @add_nuw_const_const_sub(i8 %arg) {
+; CHECK-LABEL: @add_nuw_const_const_sub(
+; CHECK-NEXT:    [[T1:%.*]] = sub i8 -128, [[ARG:%.*]]
+; CHECK-NEXT:    ret i8 [[T1]]
+;
+  %t0 = add nuw i8 %arg, 1
+  %t1 = sub i8 -127, %t0
+  ret i8 %t1
+}
+
 define i8 @add_const_const_sub_nuw(i8 %arg) {
 ; CHECK-LABEL: @add_const_const_sub_nuw(
 ; CHECK-NEXT:    [[T1:%.*]] = sub i8 -128, [[ARG:%.*]]