[PATCH] D77642: [InstCombine] exclude bitcast of ppc_fp128 in icmp signbit fold

[Sanjay Patel via Phabricator via llvm-commits]

spatel updated this revision to Diff 255708.
spatel added a comment.

Patch updated:
Add code comment to better explain the transform and PPC limitation.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D77642/new/

https://reviews.llvm.org/D77642

Files:
  llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
  llvm/test/Transforms/InstCombine/icmp.ll


Index: llvm/test/Transforms/InstCombine/icmp.ll
===================================================================
--- llvm/test/Transforms/InstCombine/icmp.ll
+++ llvm/test/Transforms/InstCombine/icmp.ll
@@ -3700,8 +3700,9 @@
 
 define i1 @signbit_bitcast_fpext_ppc_fp128(float %x) {
 ; CHECK-LABEL: @signbit_bitcast_fpext_ppc_fp128(
-; CHECK-NEXT:    [[TMP1:%.*]] = bitcast float [[X:%.*]] to i32
-; CHECK-NEXT:    [[S4:%.*]] = icmp slt i32 [[TMP1]], 0
+; CHECK-NEXT:    [[S2:%.*]] = fpext float [[X:%.*]] to ppc_fp128
+; CHECK-NEXT:    [[S3:%.*]] = bitcast ppc_fp128 [[S2]] to i128
+; CHECK-NEXT:    [[S4:%.*]] = icmp slt i128 [[S3]], 0
 ; CHECK-NEXT:    ret i1 [[S4]]
 ;
   %s2 = fpext float %x to ppc_fp128
Index: llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -2763,11 +2763,13 @@
         return new ICmpInst(Pred, X, ConstantInt::getNullValue(X->getType()));
 
     // If this is a sign-bit test of a bitcast of a casted FP value, eliminate
-    // the FP cast because the FP cast does not change the sign-bit.
+    // the FP extend/truncate because that cast does not change the sign-bit.
+    // This is true for all standard IEEE-754 types and the X86 80-bit type.
+    // The sign-bit is always the most significant bit in those types.
     const APInt *C;
     bool TrueIfSigned;
-    if (match(Op1, m_APInt(C)) && Bitcast->hasOneUse() &&
-        isSignBitCheck(Pred, *C, TrueIfSigned)) {
+    if (!BCSrcOp->getType()->isPPC_FP128Ty() && match(Op1, m_APInt(C)) &&
+        Bitcast->hasOneUse() && isSignBitCheck(Pred, *C, TrueIfSigned)) {
       if (match(BCSrcOp, m_FPExt(m_Value(X))) ||
           match(BCSrcOp, m_FPTrunc(m_Value(X)))) {
         // (bitcast (fpext/fptrunc X)) to iX) < 0 --> (bitcast X to iY) < 0


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