On Wed, 9 Nov 2022, Philipp Tomsich wrote:
> > To give a specific example that will be problematic if you go far enough
> > down
> > the road of matching MIPS64 behavior:
> >
> > long f(void)
> > {
> > int x;
> > asm("" : "=r"(x));
> > return x;
> > }
> >
> > here GCC (unlike LLVM) omits sign extension of 'x', assuming that asm output
> > must have been sign-extended to 64 bits by the programmer.
>
> In fact, with the proposed patch (but also without it), GCC will sign-extend:
> f:
> sext.w a0,a0
> ret
> .size f, .-f
I'm aware. I said "will be problematic if ...", meaning that GCC omits sign
extension when targeting MIPS64, and if you match MIPS64 behavior on RISC-V,
you'll get in that situation as well.
> To make sure that this is not just an extension to promote the int to
> long for the function return, I next added another empty asm to
> consume 'x'.
> This clearly shows that the extension is performed to postprocess the
> output of the asm-statement:
>
> f:
> # ./asm2.c:4: asm("" : "=r"(x));
> sext.w a0,a0 # x, x
> # ./asm2.c:5: asm("" : : "r"(x));
> # ./asm2.c:7: }
> ret
No, you cannot distinguish post-processing the output of the first asm vs.
pre-processing the input of the second asm. Try
asm("" : "+r"(x));
as the second asm instead, and you'll get
f:
# t.c:17: asm("" : "=r"(x));
# t.c:18: asm("" : "+r"(x));
# t.c:20: }
sext.w a0,a0 #, x
ret
If it helps, here's a Compiler Explorer link comparing with MIPS64:
https://godbolt.org/z/7eobvdKdK
Alexander
