[llvm] [ARM] Allow FP reg conversion when copying Sx to Dx (PR #147559)

[via llvm-commits]

https://github.com/eleviant updated https://github.com/llvm/llvm-project/pull/147559

>From 205ab8a252161b1839f9e5a5ea23002557f4c10c Mon Sep 17 00:00:00 2001
From: Evgeny Leviant <eleviant at accesssoftek.com>
Date: Tue, 15 Jul 2025 18:48:43 +0200
Subject: [PATCH] [ARM] Emit error message when incompatible reg is specified

At the moment the following piece of code casues undefined behaviour:
```
int a;
void b() {
  register float d2 asm("d2") =
      a;
  asm("" ::"r"(d2));
}
```
This happens because variable and register types are incompatible.
---
 llvm/lib/Target/ARM/ARMISelLowering.cpp | 13 ++++++++++++-
 llvm/test/CodeGen/ARM/bad-constraint.ll | 25 +++++++++++++++++++++++++
 2 files changed, 37 insertions(+), 1 deletion(-)
 create mode 100644 llvm/test/CodeGen/ARM/bad-constraint.ll

diff --git a/llvm/lib/Target/ARM/ARMISelLowering.cpp b/llvm/lib/Target/ARM/ARMISelLowering.cpp
index 5378ca2a485d4..f483acff411a4 100644
--- a/llvm/lib/Target/ARM/ARMISelLowering.cpp
+++ b/llvm/lib/Target/ARM/ARMISelLowering.cpp
@@ -20357,6 +20357,13 @@ ARMTargetLowering::getSingleConstraintMatchWeight(
   return weight;
 }
 
+static bool isIncompatibleReg(const MCPhysReg &PR, MVT VT) {
+  if (VT == MVT::Other)
+    return false;
+  return (ARM::SPRRegClass.contains(PR) && VT != MVT::f32) ||
+         (ARM::DPRRegClass.contains(PR) && VT != MVT::f64);
+}
+
 using RCPair = std::pair<unsigned, const TargetRegisterClass *>;
 
 RCPair ARMTargetLowering::getRegForInlineAsmConstraint(
@@ -20430,7 +20437,11 @@ RCPair ARMTargetLowering::getRegForInlineAsmConstraint(
   if (StringRef("{cc}").equals_insensitive(Constraint))
     return std::make_pair(unsigned(ARM::CPSR), &ARM::CCRRegClass);
 
-  return TargetLowering::getRegForInlineAsmConstraint(TRI, Constraint, VT);
+  auto RCP = TargetLowering::getRegForInlineAsmConstraint(TRI, Constraint, VT);
+  if (RCP.first)
+    if (isIncompatibleReg(RCP.first, VT))
+      return {0, nullptr};
+  return RCP;
 }
 
 /// LowerAsmOperandForConstraint - Lower the specified operand into the Ops
diff --git a/llvm/test/CodeGen/ARM/bad-constraint.ll b/llvm/test/CodeGen/ARM/bad-constraint.ll
new file mode 100644
index 0000000000000..9b8fcd576db5d
--- /dev/null
+++ b/llvm/test/CodeGen/ARM/bad-constraint.ll
@@ -0,0 +1,25 @@
+; RUN: not llc -filetype=obj %s -o /dev/null 2>&1 | FileCheck %s
+; CHECK:      error: couldn't allocate input reg for constraint '{d2}'
+; CHECK-NEXT: error: couldn't allocate input reg for constraint '{s2}'
+
+target datalayout = "e-m:e-p:32:32-Fi8-i64:64-v128:64:128-a:0:32-n32-S64"
+target triple = "armv8a-unknown-linux-gnueabihf"
+
+ at a = local_unnamed_addr global i32 0, align 4
+
+define void @_Z1bv() local_unnamed_addr {
+entry:
+  %0 = load i32, ptr @a, align 4
+  %conv = sext i32 %0 to i64
+  tail call void asm sideeffect "", "{d2}"(i64 %conv)
+  ret void
+}
+
+define void @_Z1cv() local_unnamed_addr {
+entry:
+  %0 = load i32, ptr @a, align 4
+  %conv = sext i32 %0 to i64
+  tail call void asm sideeffect "", "{s2}"(i64 %conv)
+  ret void
+}
+