[Bug target/54239] Not able to generate "prefetch" (prefetch read) instruction using -m3dnow or -mprfchw

[venkataramanan.kumar at amd dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54239

--- Comment #2 from Venkataramanan <venkataramanan.kumar at amd dot com> 2012-08-13 13:51:08 UTC ---
(In reply to comment #1)
> Both in 4.7 (which is before the prfchw changes) and 4.8 with -m32 -m3dnow and
> -m32 -m3dnow -mno-sse I get prefetch + prefetchw insn, which looks ok to me.
> -mno-mmx I think disables 3dnow too, so you get no prefetch insns in that case
> (which is also fine).  -mprfchw implies the SSE prefetches and PRFCHW CPUID
> 0x80000001 ecx bit 8 doesn't imply the prefetch insn, just prefetchw, so it is
> correct that with -m32 -mprfchw prefetchnta + prefetchw is generated.
> So, where exactly do you see a bug?

Hi Jakub,

> -mprfchw implies the SSE prefetches and PRFCHW CPUID
> 0x80000001 ecx bit 8 doesn't imply the prefetch insn, just prefetchw, so it is
> correct that with -m32 -mprfchw prefetchnta + prefetchw is generated.
> So, where exactly do you see a bug

As per AMD cpuid manual, 0x80000001 ecx bit 8 impiles both prefetch and
prefetchw.  

http://blogs.amd.com/developer/2010/08/18/3dnow-deprecated/