[llvm] [SelectionDAG][ARM] Propagate fast math flags in visitBRCOND (PR #156647)

[via llvm-commits]

https://github.com/paperchalice created https://github.com/llvm/llvm-project/pull/156647

Factor out from #151275.

>From c1b9fbf245cefe5122a4fe6e0548ef571152f459 Mon Sep 17 00:00:00 2001
From: PaperChalice <liujunchang97 at outlook.com>
Date: Wed, 3 Sep 2025 19:29:19 +0800
Subject: [PATCH] [SelectionDAG][ARM] Propagate fast math flags in visitBRCOND

---
 llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp |  8 ++++----
 llvm/lib/Target/ARM/ARMISelLowering.cpp       | 11 +++++++----
 2 files changed, 11 insertions(+), 8 deletions(-)

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 15d7e7626942d..cc0bb950bae00 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -19341,13 +19341,13 @@ SDValue DAGCombiner::visitBRCOND(SDNode *N) {
   // MachineBasicBlock CFG, which is awkward.
 
   // fold a brcond with a setcc condition into a BR_CC node if BR_CC is legal
-  // on the target.
+  // on the target, also copy fast math flags.
   if (N1.getOpcode() == ISD::SETCC &&
       TLI.isOperationLegalOrCustom(ISD::BR_CC,
                                    N1.getOperand(0).getValueType())) {
-    return DAG.getNode(ISD::BR_CC, SDLoc(N), MVT::Other,
-                       Chain, N1.getOperand(2),
-                       N1.getOperand(0), N1.getOperand(1), N2);
+    return DAG.getNode(ISD::BR_CC, SDLoc(N), MVT::Other, Chain,
+                       N1.getOperand(2), N1.getOperand(0), N1.getOperand(1), N2,
+                       N1->getFlags());
   }
 
   if (N1.hasOneUse()) {
diff --git a/llvm/lib/Target/ARM/ARMISelLowering.cpp b/llvm/lib/Target/ARM/ARMISelLowering.cpp
index 830156359e9e8..c4fcfe21e15f5 100644
--- a/llvm/lib/Target/ARM/ARMISelLowering.cpp
+++ b/llvm/lib/Target/ARM/ARMISelLowering.cpp
@@ -5570,7 +5570,7 @@ static void expandf64Toi32(SDValue Op, SelectionDAG &DAG,
   llvm_unreachable("Unknown VFP cmp argument!");
 }
 
-/// OptimizeVFPBrcond - With -enable-unsafe-fp-math, it's legal to optimize some
+/// OptimizeVFPBrcond - With nnan, it's legal to optimize some
 /// f32 and even f64 comparisons to integer ones.
 SDValue
 ARMTargetLowering::OptimizeVFPBrcond(SDValue Op, SelectionDAG &DAG) const {
@@ -5712,9 +5712,12 @@ SDValue ARMTargetLowering::LowerBR_CC(SDValue Op, SelectionDAG &DAG) const {
     return DAG.getNode(ARMISD::BRCOND, dl, MVT::Other, Chain, Dest, ARMcc, Cmp);
   }
 
-  if (getTargetMachine().Options.UnsafeFPMath &&
-      (CC == ISD::SETEQ || CC == ISD::SETOEQ ||
-       CC == ISD::SETNE || CC == ISD::SETUNE)) {
+  if (SDNodeFlags Flags = Op->getFlags();
+      (getTargetMachine().Options.UnsafeFPMath || Flags.hasNoNaNs()) &&
+      (DAG.getDenormalMode(MVT::f32) == DenormalMode::getIEEE() &&
+      DAG.getDenormalMode(MVT::f64) == DenormalMode::getIEEE()) &&
+      (CC == ISD::SETEQ || CC == ISD::SETOEQ || CC == ISD::SETNE ||
+       CC == ISD::SETUNE)) {
     if (SDValue Result = OptimizeVFPBrcond(Op, DAG))
       return Result;
   }