From: Jeff Law <law@redhat.com>
To: Stefan Kanthak <stefan.kanthak@nexgo.de>, gcc-patches@gcc.gnu.org
Subject: Re: [PATCH] Better __ashlDI3, __ashrDI3 and __lshrDI3 functions, plus fixed __bswapsi2 function
Date: Tue, 10 Nov 2020 16:48:10 -0700 [thread overview]
Message-ID: <c5e5881c-116d-2ecc-81ea-e86475618529@redhat.com> (raw)
In-Reply-To: <116F1589A8244FB494091BCD4E6398AB@H270>
On 11/10/20 10:59 AM, Stefan Kanthak via Gcc-patches wrote:
> The implementation of the __ashlDI3(), __ashrDI3() and __lshrDI3() functions
> is rather bad, it yields bad machine code at least on i386 and AMD64.
> Since GCC knows how to shift integers twice the register size these functions
> can be written as one-liners.
>
> The implementation of the __bswapsi2() function uses SIGNED instead of
> unsigned mask values; cf. __bswapdi2()
>
> Stefan Kanthak
>
> libgcc2.diff
>
> --- -/libgcc/libgcc2.h
> +++ +/libgcc/libgcc2.h
> @@ -391,7 +391,7 @@
> extern DWtype __negdi2 (DWtype);
> #endif
>
> -extern DWtype __lshrdi3 (DWtype, shift_count_type);
> +extern UDWtype __lshrdi3 (UDWtype, shift_count_type);
> extern DWtype __ashldi3 (DWtype, shift_count_type);
> extern DWtype __ashrdi3 (DWtype, shift_count_type);
>
> --- -/libgcc/libgcc2.c
> +++ +/libgcc/libgcc2.c
> @@ -398,30 +398,10 @@
> /* Unless shift functions are defined with full ANSI prototypes,
> parameter b will be promoted to int if shift_count_type is smaller than an int. */
> #ifdef L_lshrdi3
> -DWtype
> -__lshrdi3 (DWtype u, shift_count_type b)
> +UDWtype
> +__lshrdi3 (UDWtype u, shift_count_type b)
As has been pointed out, you can't implement these routines by doing the
operation in the same mode as the argument. It's potentially
self-recursive.
THe whole point of these routines is to provide double-word capabilities
on targets that don't have intrinsic double-word capabilities. That's
why they're written in a non-obvious way using word sized operations.
> \f
> @@ -486,10 +425,10 @@
> SItype
> __bswapsi2 (SItype u)
> {
> - return ((((u) & 0xff000000) >> 24)
> - | (((u) & 0x00ff0000) >> 8)
> - | (((u) & 0x0000ff00) << 8)
> - | (((u) & 0x000000ff) << 24));
> + return ((((u) & 0xff000000u) >> 24)
> + | (((u) & 0x00ff0000u) >> 8)
> + | (((u) & 0x0000ff00u) << 8)
> + | (((u) & 0x000000ffu) << 24));
What's the point of this change? I'm not sure how the signedness of the
constant really matters here.
jeff
next prev parent reply other threads:[~2020-11-10 23:48 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-11-10 17:59 Stefan Kanthak
2020-11-10 18:08 ` Eric Botcazou
2020-11-10 19:44 ` Stefan Kanthak
2020-11-10 20:14 ` Jakub Jelinek
2020-11-10 21:09 ` Jeff Law
2020-11-10 21:17 ` Jakub Jelinek
2020-11-10 23:42 ` Jeff Law
2020-11-24 0:21 ` Jeff Law
2020-11-10 22:12 ` Jeff Law
2020-11-10 22:01 ` Jeff Law
2020-11-11 0:00 ` Andreas Schwab
2020-11-24 13:57 ` Stefan Kanthak
2020-11-24 14:34 ` Andreas Schwab
2020-11-24 15:40 ` Stefan Kanthak
2020-11-24 15:49 ` Andreas Schwab
2020-11-25 18:53 ` Jeff Law
2020-11-25 20:22 ` Stefan Kanthak
2020-11-25 20:42 ` Jakub Jelinek
2020-11-25 21:22 ` Stefan Kanthak
2020-11-25 22:06 ` Jeff Law
2020-11-25 23:06 ` Stefan Kanthak
2020-11-25 20:44 ` Jeff Law
2020-11-10 18:09 ` Jakub Jelinek
2020-11-10 18:32 ` Jeff Law
2020-11-10 18:26 ` Jakub Jelinek
2020-11-10 23:48 ` Jeff Law [this message]
2020-11-10 23:53 ` Jakub Jelinek
2020-11-11 8:33 ` Stefan Kanthak
2020-11-11 9:55 ` Jakub Jelinek
2020-11-11 17:54 ` Joseph Myers
2020-11-23 23:01 ` Jeff Law
2020-11-30 1:06 ` Jeff Law
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