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Date: Fri, 20 Feb 2015 16:48:12 +0100
From: Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH 2/2] milkymist-pfpu: fix GCC 5.0.0
 aggressive-loop-optimizations warning
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Michael Walle <michael@walle.cc>, Paolo Bonzini <paolo.bonzini@gmail.com>, qemu-devel@nongnu.org

2015-02-20 15:55+0100, Paolo Bonzini:
> 
> 
> On 20/02/2015 15:52, Michael Walle wrote:
> >>>
> >>> -            i = 0;
> >>> -            while (pfpu_decode_insn(s)) {
> >>> -                /* decode at most MICROCODE_WORDS instructions */
> >>> -                if (i++ >= MICROCODE_WORDS) {
> >>
> >> Isn't the fix just to say "++i" instead of "i++"?
> > 
> > In the first run, s->regs[R_PC] may have any value, therefore the "insn
> > = s->microcode[pc]" from above may access out of bounds.
> 
> Then should pfpu_decode_insn access s->microcode[pc & (MICROCODE_WORDS -
> 1)]?  That's likely what happens in hardware, and the purpose of the
> error is just to avoid an infinite loop in device code.

http://www.milkymist.org/socdoc/pfpu.pdf is dead, but the source isn't:
https://github.com/m-labs/milkymist/blob/master/cores/pfpu/doc/pfpu.tex

I don't see the PC register mentioned in interface, so hiding it would
probably be a good start.