Clang MIPS: float register constraints for inline asm deviate from gcc

[shravanrn]

clang and gcc seem to have different behavior of floating point constraints for inline asm targetting MIPS. Consider this simple example

// gcc compiles this but clang does not
void read_float(float* p) {
    float result = *p;
    __asm__("" ::"r"(result));
}

The following code compiles fine on gcc when targeting MIPS, but clang gives a compile error couldn't allocate input reg for constraint 'r'

Clang requires the register constraint here to be "f" not "r". However applying an "f" constraint, causes gcc to fail compilation with the error error: impossible constraint in 'asm' while clang compiles the code fine.

// clang compiles this but gcc does not
void read_float(float* p) {
    float result = *p;
    __asm__("" ::"f"(result));
}

Here is a simple demo of the issue: https://godbolt.org/z/WGWxd3ejo
Test with "mips gcc 13.1.0" and "mips clang 16.0.0"

(This bug was discovered as part of the wasm2c project WebAssembly/wabt#2266)

[llvmbot]

@llvm/issue-subscribers-backend-mips

[pinskia]

Note the f constraint works with GCC if enable hard float (that is -mhard-float option). The godbolt's mips gcc defaults to soft-float (--with-float=soft on the configure line).

[brad0]

cc @wzssyqa @yingopq

[wzssyqa]

Should we consider it as a bug of gcc or clang ;)
I think that we should support "f" for soft float, and map it to "r".

[pinskia]

Should we consider it as a bug of gcc or clang ;) I think that we should support "f" for soft float, and map it to "r".
It is not a bug in either, just the default configure options for GCC vs clang on godbolt. GCC defaults to soft-float while clang defaults to hard float.

Note with -msoft-float on clang, it is rejected too. Though there is an internal compiler error and that is definitely a bug.

[wzssyqa]

Ohh, I know. Let me have a try.

[wzssyqa]

This PR fixes half of this problem: no-ICE if f is used for soft-float.

We also need to support r for soft-float.

[wzssyqa]

#79116

can solve the whole of problem now.