Re: [PATCH] sha: prevent removal of memset as dead store insha1_update()

[Andi Kleen]

> Instead of barrier(), this works with gcc-3.2.3 up to gcc-4.4.3
> for the purpose of making the memset() not disappear:
> 
> 	{
> 		struct s { char c[n]; };
> 		asm("" : : "m"(*(struct s *)p));
> 	}
> 
> Every byte in the [p,p+n[ range must be used. If you only use the
> first byte, via e.g. asm("" :: "m"(*(char*)p)), then the compiler
> _will_ skip scrubbing bytes beyond the first.
> 
> An explicit loop that uses each byte individually also works, but
> results in awful code with older compilers.

I suspect it would be worth asking for a __builtin for this.
iirc Visual C already has one.

BTW I remember finding a few more such cases in random user space
code in past code review.

-Andi
-- 
ak@xxxxxxxxxxxxxxx -- Speaking for myself only.
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