Re: [PATCH v2 00/10] x86: use gcc 6+ asm flag output feature

From: H. Peter Anvin
Date: Wed Jun 08 2016 - 17:31:43 EST


On June 8, 2016 12:38:36 PM PDT, "H. Peter Anvin" <hpa@xxxxxxxxxxxxxxx> wrote:
>From: "H. Peter Anvin" <hpa@xxxxxxxxx>
>
>gcc 6+ has the ability to let flags (actually, conditions, which are
>specific combinations of flags) to be used directly as asm() outputs.
>The syntax for that is "=@cc<cc>" where <cc> is the same set of
>letters that would be used in a j<cc> or set<cc> instruction
>(e.g. "=@ccz" to test the ZF flag.)
>
>This patchset by itself reduces the size of the x86-64 kernel by
>0.12%, from a baseline of 4.7-rc2 built with gcc 6.1 (first line is
>with the patchset, the second one is without):
>
> text data bss dec hex filename
>
>68245656 41004339 20533248 129783243 7bc55cb
>o.i386-allconfig/vmlinux
>68355716 41008499 20533248 129897463 7be13f7
>o.i386-allconfig/vmlinux
>
>127384005 129742359 38150144 295276508 11998fdc
>o.x86_64-allconfig/vmlinux
>127538765 129742295 38150144 295431204 119bec24
>o.x86_64-allconfig/vmlinux
>
>v2: fix a conflict between <linux/random.h> and <asm/archrandom.h>
> discovered by Ingo Molnar. There are a few places in x86-specific
> code where we need all of <arch/archrandom.h> even when
> xCONFIG_ARCH_RANDOM is disabled, so <linux/random.h> does not
> suffice. Only the <asm/archrandom.h> patch has been changed;
> since it is a little bit different from the others I have moved it
> to the end.

I should probably clarify one thing: the output type *must* be bool for the flags output. Fortunately, not only does it also work for the older compilers, but it sometimes allows GCC to generate better code, which is why I have made the change unconditionally.
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