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Date: Wed, 9 Nov 2022 20:21:33 +0300 (MSK)
From: Alexander Monakov <amonakov@ispras.ru>
To: Philipp Tomsich <philipp.tomsich@vrull.eu>
cc: gcc-patches@gcc.gnu.org, Vineet Gupta <vineetg@rivosinc.com>, 
    Kito Cheng <kito.cheng@gmail.com>, Jojo R <rjiejie@linux.alibaba.com>, 
    Jeff Law <jlaw@ventanamicro.com>
Subject: Re: [PATCH] riscv: implement TARGET_MODE_REP_EXTENDED
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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