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
Date: Wed, 9 Nov 2022 20:21:33 +0300 (MSK) [thread overview]
Message-ID: <b82bf348-d167-43e2-564c-44f5da2eacf@ispras.ru> (raw)
In-Reply-To: <CAAeLtUCESGy=xZVjPP5wvFA2RqKChc4LHib3xXOGrmhVde8R=Q@mail.gmail.com>
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
next prev parent reply other threads:[~2022-11-09 17:21 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-09-05 21:44 Philipp Tomsich
2022-09-06 11:39 ` Alexander Monakov
2022-09-16 23:48 ` Jeff Law
2022-09-17 7:59 ` Palmer Dabbelt
2022-11-04 23:00 ` Philipp Tomsich
2022-11-05 6:16 ` [PATCH] [PHIOPT] Add A ? B + CST : B match and simplify optimizations Zhongyunde
2022-11-05 6:34 ` Andrew Pinski
2022-11-05 9:03 ` Zhongyunde
2022-11-08 14:58 ` Richard Biener
2022-11-08 15:51 ` 钟云德
2022-11-09 8:00 ` Richard Biener
2022-11-07 13:55 ` [PATCH] riscv: implement TARGET_MODE_REP_EXTENDED Alexander Monakov
2022-11-08 23:45 ` Philipp Tomsich
2022-11-09 17:21 ` Alexander Monakov [this message]
2022-11-20 16:09 ` Jeff Law
2022-11-21 13:49 ` Alexander Monakov
2022-11-21 14:56 ` Jeff Law
2022-11-21 15:33 ` Alexander Monakov
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