[ARM] Emit error message when incompatible reg is specified (#147559)

At the moment the following piece of code causes undefined behavior:
```
int a;
void b() {
   register float d2 asm("d2") = a;
   asm("" ::"r"(d2));
}
```
This happens because variable and register types are incompatible.

Author: eleviant

llvm/lib/Target/ARM/ARMISelLowering.cpp

@@ -20347,6 +20347,13 @@ ARMTargetLowering::getSingleConstraintMatchWeight(
   return weight;
 }
 
+static bool isIncompatibleReg(const MCPhysReg &PR, MVT VT) {
+  if (PR == 0 || VT == MVT::Other)
+    return false;
+  return (ARM::SPRRegClass.contains(PR) && VT != MVT::f32 && VT != MVT::i32) ||
+         (ARM::DPRRegClass.contains(PR) && VT != MVT::f64);
+}
+
 using RCPair = std::pair<unsigned, const TargetRegisterClass *>;
 
 RCPair ARMTargetLowering::getRegForInlineAsmConstraint(
@@ -20420,7 +20427,10 @@ 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 (isIncompatibleReg(RCP.first, VT))
+    return {0, nullptr};
+  return RCP;
 }
 
 /// LowerAsmOperandForConstraint - Lower the specified operand into the Ops

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"
+
+@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
+}
+

llvm/test/CodeGen/ARM/inlineasm-int-to-float.ll

@@ -0,0 +1,17 @@
+; RUN: llc -filetype=asm %s -o - | FileCheck %s
+
+; CHECK:      movw r0, :lower16:a
+; CHECK-NEXT: movt r0, :upper16:a
+; CHECK-NEXT: vldr s6, [r0]
+
+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"
+
+@a = local_unnamed_addr global i32 0, align 4
+
+define void @_Z1dv() local_unnamed_addr {
+entry:
+  %0 = load i32, ptr @a, align 4
+  tail call void asm sideeffect "", "{s6}"(i32 %0)
+  ret void
+}