[llvm] [ARM] Emit a warning when the hard-float ABI is enabled but can't be used. (PR #111334)

[Peter Smith via llvm-commits]

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@@ -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)
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smithp35 wrote:

When I run this on a test program I get a couple of warnings
> bin/clang --target=arm-none-eabihf -march=armv7-r nofp.c -c -mfloat-abi=hard -ffunction-sections
clang: warning: '-mfloat-abi=hard': selected processor lacks floating point registers [-Wunsupported-abi]
The hard-float ABI can't be used for a target that doesn't support floating-point (ignoring float-abi)
The hard-float ABI can't be used for a target that doesn't support floating-point (ignoring float-abi)

I think this is because this constructor is run once per ARMSubtarget. Tracing this back to getSubtargetImpl where ARMSubtargets are created https://github.com/llvm/llvm-project/blob/main/llvm/lib/Target/ARM/ARMTargetMachine.cpp#L280

It looks like that might be a better place to put this diagnostic. We may also want to see if some additional information like the function name can be used, possibly something from the Subtarget if the output is human readable.

https://github.com/llvm/llvm-project/pull/111334