[InstCombine] Allow overflowing selects to work on commutative arguments

[AreaZR]

This came up when working on a patch for systemd, where the code would not simplify because the code read:

unsigned int u(unsigned int x, unsigned int y)
{
    unsigned int z;
    if (__builtin_uadd_overflow(x, y, &z))
         return UINT_MAX; /* indicate overflow */
    return x + y;
}

Instead of:

unsigned int u(unsigned int x, unsigned int y)
{
    unsigned int z;
    if (__builtin_uadd_overflow(x, y, &z))
         return UINT_MAX; /* indicate overflow */
    return z;
}

[llvmbot]

@llvm/pr-subscribers-llvm-transforms

Author: AtariDreams (AtariDreams)

Changes

This came up when working on a patch for systemd, where the code would not simplify because the code read:

unsigned int u(unsigned int x, unsigned int y)
{
    unsigned int z;
    if (__builtin_uadd_overflow(x, y, &z))
         return UINT_MAX; /* indicate overflow */
    return x + y;
}

Instead of:

unsigned int u(unsigned int x, unsigned int y)
{
    unsigned int z;
    if (__builtin_uadd_overflow(x, y, &z))
         return UINT_MAX; /* indicate overflow */
    return z;
}

---

Full diff: https://github.com/llvm/llvm-project/pull/90812.diff

2 Files Affected:

• (modified) llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp (+17-2)
• (modified) llvm/test/Transforms/InstCombine/overflow_to_sat.ll (+12)

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
index 8818369e79452b..d10adb1667c020 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
@@ -2075,12 +2075,27 @@ foldOverflowingAddSubSelect(SelectInst &SI, InstCombiner::BuilderTy &Builder) {
   Value *FalseVal = SI.getFalseValue();
 
   WithOverflowInst *II;
-  if (!match(CondVal, m_ExtractValue<1>(m_WithOverflowInst(II))) ||
-      !match(FalseVal, m_ExtractValue<0>(m_Specific(II))))
+  if (!match(CondVal, m_ExtractValue<1>(m_WithOverflowInst(II))))
     return nullptr;
 
   Value *X = II->getLHS();
   Value *Y = II->getRHS();
+  if (!match(FalseVal, m_ExtractValue<0>(m_Specific(II)))) {
+    switch (II->getIntrinsicID()) {
+    case Intrinsic::uadd_with_overflow:
+    case Intrinsic::sadd_with_overflow:
+      if (!match(FalseVal, m_c_Add(m_Specific(X), m_Specific(Y))))
+        return nullptr;
+      break;
+    case Intrinsic::usub_with_overflow:
+    case Intrinsic::ssub_with_overflow:
+      if (!match(FalseVal, m_Sub(m_Specific(X), m_Specific(Y))))
+        return nullptr;
+      break;
+    default:
+      return nullptr;
+    }
+  }
 
   auto IsSignedSaturateLimit = [&](Value *Limit, bool IsAdd) {
     Type *Ty = Limit->getType();
diff --git a/llvm/test/Transforms/InstCombine/overflow_to_sat.ll b/llvm/test/Transforms/InstCombine/overflow_to_sat.ll
index 568ac77d6b88dd..eaa247b42a37b9 100644
--- a/llvm/test/Transforms/InstCombine/overflow_to_sat.ll
+++ b/llvm/test/Transforms/InstCombine/overflow_to_sat.ll
@@ -13,6 +13,18 @@ define i32 @uadd(i32 %x, i32 %y) {
   ret i32 %s
 }
 
+define i32 @uadd_cumm(i32 %x, i32 %y) {
+; CHECK-LABEL: @uadd_cumm(
+; CHECK-NEXT:    [[S:%.*]] = call i32 @llvm.uadd.sat.i32(i32 [[X:%.*]], i32 [[Y:%.*]])
+; CHECK-NEXT:    ret i32 [[S]]
+;
+  %ao = tail call { i32, i1 } @llvm.uadd.with.overflow.i32(i32 %x, i32 %y)
+  %o = extractvalue { i32, i1 } %ao, 1
+  %a = add i32 %y, %x
+  %s = select i1 %o, i32 -1, i32 %a
+  ret i32 %s
+}
+
 define i32 @usub(i32 %x, i32 %y) {
 ; CHECK-LABEL: @usub(
 ; CHECK-NEXT:    [[S:%.*]] = call i32 @llvm.usub.sat.i32(i32 [[X:%.*]], i32 [[Y:%.*]])

[nikic]

This looks reasonable to me.

[topperc]

Does the case with add i32 %x, %y here already work before this patch or do we need a test for it too?

[AreaZR]

Does the case with add i32 %x, %y here already work before this patch or do we need a test for it too?

It already works before this patch. I tested it.

[nikic]

Does it? https://llvm.godbolt.org/z/3Wb4ao3e3

[topperc]

Is this tested? I don't see any sub instructions in your new tests

[AreaZR]

Is this tested? I don't see any sub instructions in your new tests

sub already works as intended.

[nikic]

@AtariDreams Please never mark conversations as resolved yourself. Your resolutions are wrong half of the time. The reviewer will resolve the conversation if appropriate.