[llvm] [LegalizeDAG] PromoteNode FABS: Override promotion with clear sign bit (PR #106076)

[via llvm-commits]

https://github.com/v01dXYZ updated https://github.com/llvm/llvm-project/pull/106076

>From b42e049ce8341094bc5dc04f2d54211caa9ac2bc Mon Sep 17 00:00:00 2001
From: v01dxyz <v01dxyz at v01d.xyz>
Date: Mon, 26 Aug 2024 15:16:29 +0200
Subject: [PATCH] [LegalizeDAG] PromoteNode FABS: Override promotion with clear
 sign bit

Conditionally to the same sized integer type being legal.

For example, for f16, if i16 is legal, bitcast to i16, clear the sign
bit and bitcast back to f16.

Adapted from a suggested patch from @arsenm (Matthew.Arsenault at amd.com):

https://github.com/llvm/llvm-project/issues/104915#issuecomment-2301660006
---
 llvm/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp | 22 ++++++++++++++++++-
 1 file changed, 21 insertions(+), 1 deletion(-)

diff --git a/llvm/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp
index 74e3a898569bea..fe8fb5d45676b5 100644
--- a/llvm/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp
@@ -5576,6 +5576,27 @@ void SelectionDAGLegalize::PromoteNode(SDNode *Node) {
     Results.push_back(Tmp2.getValue(1));
     break;
   }
+  case ISD::FABS: {
+    // if we can convert to a same sized int value, we bitcast to it,
+    // clear the sign bit and bitcast back to the original type.
+    EVT IntVT = OVT.changeTypeToInteger();
+    if (TLI.isTypeLegal(IntVT)) {
+      SDValue Cast = DAG.getNode(ISD::BITCAST, dl, IntVT, Node->getOperand(0));
+
+      APInt SignMask = APInt::getSignMask(IntVT.getScalarSizeInBits());
+
+      // Mask = ~(1 << (Size-1))
+      SDValue Mask = DAG.getConstant(SignMask, dl, IntVT);
+
+      SDValue And = DAG.getNode(ISD::AND, dl, IntVT, Cast, Mask);
+
+      SDValue Result = DAG.getNode(ISD::BITCAST, dl, OVT, And);
+      Results.push_back(Result);
+      break;
+    }
+
+    [[fallthrough]];
+  }
   case ISD::FFLOOR:
   case ISD::FCEIL:
   case ISD::FRINT:
@@ -5597,7 +5618,6 @@ void SelectionDAGLegalize::PromoteNode(SDNode *Node) {
   case ISD::FLOG:
   case ISD::FLOG2:
   case ISD::FLOG10:
-  case ISD::FABS:
   case ISD::FEXP:
   case ISD::FEXP2:
   case ISD::FEXP10: