[llvm] [InstCombine] Fold copysign of selects from sign comparison to sign operand (PR #85627)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: Krishna Narayanan (Krishna-13-cyber)

<details>
<summary>Changes</summary>

This is currently under development and needs some assistance/review to go about solving this.
This PR intends to solve this issue #<!-- -->64884.
Blocker: 
I pattern matched the most probable scenario according to me. This approach doesn't give the desired outcome of folding instructions and fails tests. 
Any lead on where I am going wrong/could improve?

---
Full diff: https://github.com/llvm/llvm-project/pull/85627.diff


2 Files Affected:

- (modified) llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp (+45) 
- (modified) llvm/test/Transforms/InstCombine/fcmp.ll (+16) 


``````````diff
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
index ee76a6294428b3..49e10646eb5d15 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
@@ -2790,6 +2790,47 @@ static Instruction *foldSelectWithFCmpToFabs(SelectInst &SI,
   return ChangedFMF ? &SI : nullptr;
 }
 
+// Canonicalize select with fcmp -> select
+static Instruction *foldSelectWithFCmp(SelectInst &SI, InstCombinerImpl &IC) {
+  /* From
+    %4 = fcmp olt float %1, 0.000000e+00
+    %5 = and i1 %4, %0
+    %6 = select i1 %5, float -1.000000e+00, float 1.000000e+00
+  */
+  /* To
+   %4 = select i1 %0, float %1, float 1.000000e+00
+  */
+  Value *CondVal = SI.getCondition();
+  Value *TrueVal = SI.getTrueValue();
+  Value *FalseVal = SI.getFalseValue();
+  Value *One = Constant::getAllOnesValue(FalseVal->getType());
+  Value *X, *C, *Op;
+  const APFloat *A, *E;
+  CmpInst::Predicate Pred;
+  for (bool Swap : {false, true}) {
+    if (Swap)
+      std::swap(TrueVal, FalseVal);
+    if (match(&SI, (m_Value(CondVal), m_APFloat(A), m_APFloat(E)))) {
+      if (!match(TrueVal, m_APFloatAllowUndef(A)) &&
+          !match(FalseVal, m_APFloatAllowUndef(E)))
+        return nullptr;
+      if (!match(CondVal, m_And(m_FCmp(Pred, m_Specific(X), m_PosZeroFP()),
+                                m_Value(C))) &&
+          (X->hasOneUse() && C->hasOneUse()))
+        return nullptr;
+      if (!A->isNegative() && E->isNegative())
+        return nullptr;
+      if (!Swap && (Pred == FCmpInst::FCMP_OLT)) {
+        return SelectInst::Create(C, X, One);
+      }
+      if (Swap && (Pred == FCmpInst::FCMP_OGT)) {
+        return SelectInst::Create(C, X, One);
+      }
+    }
+  }
+  return nullptr;
+}
+
 // Match the following IR pattern:
 //   %x.lowbits = and i8 %x, %lowbitmask
 //   %x.lowbits.are.zero = icmp eq i8 %x.lowbits, 0
@@ -3508,6 +3549,10 @@ Instruction *InstCombinerImpl::visitSelectInst(SelectInst &SI) {
   if (Instruction *Fabs = foldSelectWithFCmpToFabs(SI, *this))
     return Fabs;
 
+  // Fold selecting to ffold.
+  if (Instruction *Ffold = foldSelectWithFCmp(SI, *this))
+    return Ffold;
+
   // See if we are selecting two values based on a comparison of the two values.
   if (ICmpInst *ICI = dyn_cast<ICmpInst>(CondVal))
     if (Instruction *Result = foldSelectInstWithICmp(SI, ICI))
diff --git a/llvm/test/Transforms/InstCombine/fcmp.ll b/llvm/test/Transforms/InstCombine/fcmp.ll
index 159c84d0dd8aa9..574fb1f9417742 100644
--- a/llvm/test/Transforms/InstCombine/fcmp.ll
+++ b/llvm/test/Transforms/InstCombine/fcmp.ll
@@ -1284,3 +1284,19 @@ define <1 x i1> @bitcast_1vec_eq0(i32 %x) {
   %cmp = fcmp oeq <1 x float> %f, zeroinitializer
   ret <1 x i1> %cmp
 }
+
+define float @copysign_conditional(i1 noundef zeroext %0, float %1, float %2) {
+; CHECK-LABEL: define float @copysign_conditional(
+; CHECK-SAME: i1 noundef zeroext [[TMP0:%.*]], float [[TMP1:%.*]], float [[TMP2:%.*]]) {
+; CHECK-NEXT:    [[TMP4:%.*]] = fcmp olt float [[TMP1]], 0.000000e+00
+; CHECK-NEXT:    [[TMP5:%.*]] = and i1 [[TMP4]], [[TMP0]]
+; CHECK-NEXT:    [[TMP6:%.*]] = select i1 [[TMP5]], float -1.000000e+00, float 1.000000e+00
+; CHECK-NEXT:    [[TMP7:%.*]] = tail call float @llvm.copysign.f32(float [[TMP2]], float [[TMP6]])
+; CHECK-NEXT:    ret float [[TMP7]]
+;
+  %4 = fcmp olt float %1, 0.000000e+00
+  %5 = and i1 %4, %0
+  %6 = select i1 %5, float -1.000000e+00, float 1.000000e+00
+  %7 = tail call float @llvm.copysign.f32(float %2, float %6)
+  ret float %7
+}
\ No newline at end of file

``````````

</details>


https://github.com/llvm/llvm-project/pull/85627