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

Currently, compiling for eabihf 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 #110383.

Author: chrisnc

clang/test/Driver/arm-float-abi-lto.c

@@ -4,7 +4,7 @@
 
 // RUN: %clang --target=arm-none-eabi -mcpu=cortex-m33 -mfloat-abi=hard -O1 %s -flto=full -c -o %t.call_full.bc -DCALL_LIB
 // RUN: %clang --target=arm-none-eabi -mcpu=cortex-m33 -mfloat-abi=hard -O1 %s -flto=full -c -o %t.define_full.bc -DDEFINE_LIB
-// RUN: llvm-lto2 run -o %t.lto_full -save-temps %t.call_full.bc %t.define_full.bc \
+// RUN: llvm-lto2 run --mcpu=cortex-m33 --float-abi=hard -o %t.lto_full -save-temps %t.call_full.bc %t.define_full.bc \
 // RUN:  -r %t.call_full.bc,fn,px \
 // RUN:  -r %t.call_full.bc,fwrite,l \
 // RUN:  -r %t.call_full.bc,putchar,l \
@@ -16,7 +16,7 @@
 
 // RUN: %clang --target=arm-none-eabi -mcpu=cortex-m33 -mfloat-abi=hard -O1 %s -flto=thin -c -o %t.call_thin.bc -DCALL_LIB
 // RUN: %clang --target=arm-none-eabi -mcpu=cortex-m33 -mfloat-abi=hard -O1 %s -flto=thin -c -o %t.define_thin.bc -DDEFINE_LIB
-// RUN: llvm-lto2 run -o %t.lto_thin -save-temps %t.call_thin.bc %t.define_thin.bc \
+// RUN: llvm-lto2 run --mcpu=cortex-m33 --float-abi=hard -o %t.lto_thin -save-temps %t.call_thin.bc %t.define_thin.bc \
 // RUN:  -r %t.call_thin.bc,fn,px \
 // RUN:  -r %t.call_thin.bc,fwrite,l \
 // RUN:  -r %t.call_thin.bc,putchar,l \

llvm/lib/Target/ARM/ARMTargetMachine.cpp

@@ -309,11 +309,15 @@ ARMBaseTargetMachine::getSubtargetImpl(const Function &F) const {
     // function that reside in TargetOptions.
     resetTargetOptions(F);
     I = std::make_unique<ARMSubtarget>(TargetTriple, CPU, FS, *this, isLittle,
-                                        F.hasMinSize());
+                                       F.hasMinSize());
 
     if (!I->isThumb() && !I->hasARMOps())
       F.getContext().emitError("Function '" + F.getName() + "' uses ARM "
           "instructions, but the target does not support ARM mode execution.");
+
+    if (I->isTargetHardFloat() && !I->hasFPRegs())
+      F.getContext().emitError("The hard-float ABI is enabled, but the target "
+                               "lacks floating-point registers.");
   }
 
   return I.get();