[ARM] Emit a warning when the hard-float ABI is enabled but can't be used.

Compiling for an eabihf target with a CPU lacking floating-point
registers will silently use the soft-float ABI instead, even though the
Arm attributes section still has Tag_ABI_VFP_args: VFP registers, which
leads to silent ABI mismatches at link time.

Fixes llvm#110383.

Author: chrisnc

llvm/lib/Target/ARM/ARMISelLowering.cpp

@@ -791,7 +791,9 @@ ARMTargetLowering::ARMTargetLowering(const TargetMachine &TM,
       setAllExpand(MVT::f32);
     if (!Subtarget->hasFP64())
       setAllExpand(MVT::f64);
-  }
+  } else if (TM.Options.FloatABIType == FloatABI::Hard)
+    errs() << "The hard-float ABI can't be used for a target that "
+              "doesn't support floating-point (ignoring float-abi)\n";
 
   if (Subtarget->hasFullFP16()) {
     addRegisterClass(MVT::f16, &ARM::HPRRegClass);

llvm/test/CodeGen/ARM/eabihf-no-fpregs.ll

@@ -0,0 +1,15 @@
+; RUN: llc -asm-verbose=false --mtriple=armv7-none-eabihf --mattr=+vfp3 < %s | FileCheck %s --check-prefix=CHECK-VFP
+; RUN: llc -asm-verbose=false --mtriple=armv7-none-eabi --mattr=-fpregs < %s | FileCheck %s -check-prefix=CHECK-NOVFP
+; RUN: llc -asm-verbose=false --mtriple=armv7-none-eabihf --mattr=-fpregs < %s 2>&1 | FileCheck %s -check-prefix=CHECK-ERROR -check-prefix=CHECK-NOVFP
+
+target datalayout = "e-p:32:32:32-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-n32"
+
+; CHECK-VFP: vadd.f32
+; CHECK-ERROR: The hard-float ABI can't be used for a target that doesn't support floating-point (ignoring float-abi)
+; CHECK-NOVFP: bl __aeabi_fadd
+define float @test_fadd(float %a, float %b) #0 {
+  %r = fadd float %a, %b
+  ret float %r
+}
+
+attributes #0 = { nounwind }