commit 1328a8534bf85ba820ffb6d97e8520aa2aa13788
author Matt Arsenault <Matthew.Arsenault@amd.com> Tue Sep 19 09:14:17 2023 +0300
committer GitHub <noreply@github.com> Tue Sep 19 09:14:17 2023 +0300
tree c9ef3f0345bcdb1266cd5f4d0d063ab3e749219f
parent b9a6b28a585257c34b3a581397436eb852f80ce1

AMDGPU: Fix handling of -0 in round lowering (#65761)

llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp

diff --git a/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp b/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
index a5e8024..0c3f558 100644
--- a/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
@@ -6586,23 +6586,25 @@
   // round(x) =>
   //  t = trunc(x);
   //  d = fabs(x - t);
-  //  o = copysign(1.0f, x);
-  //  return t + (d >= 0.5 ? o : 0.0);
+  //  o = copysign(d >= 0.5 ? 1.0 : 0.0, x);
+  //  return t + o;
 
   auto T = MIRBuilder.buildIntrinsicTrunc(Ty, X, Flags);
 
   auto Diff = MIRBuilder.buildFSub(Ty, X, T, Flags);
   auto AbsDiff = MIRBuilder.buildFAbs(Ty, Diff, Flags);
-  auto Zero = MIRBuilder.buildFConstant(Ty, 0.0);
-  auto One = MIRBuilder.buildFConstant(Ty, 1.0);
+
   auto Half = MIRBuilder.buildFConstant(Ty, 0.5);
-  auto SignOne = MIRBuilder.buildFCopysign(Ty, One, X);
+  auto Cmp =
+      MIRBuilder.buildFCmp(CmpInst::FCMP_OGE, CondTy, AbsDiff, Half, Flags);
 
-  auto Cmp = MIRBuilder.buildFCmp(CmpInst::FCMP_OGE, CondTy, AbsDiff, Half,
-                                  Flags);
-  auto Sel = MIRBuilder.buildSelect(Ty, Cmp, SignOne, Zero, Flags);
+  // Could emit G_UITOFP instead
+  auto One = MIRBuilder.buildFConstant(Ty, 1.0);
+  auto Zero = MIRBuilder.buildFConstant(Ty, 0.0);
+  auto BoolFP = MIRBuilder.buildSelect(Ty, Cmp, One, Zero);
+  auto SignedOffset = MIRBuilder.buildFCopysign(Ty, BoolFP, X);
 
-  MIRBuilder.buildFAdd(DstReg, T, Sel, Flags);
+  MIRBuilder.buildFAdd(DstReg, T, SignedOffset, Flags);
 
   MI.eraseFromParent();
   return Legalized;