Re: [Qemu-devel] [PATCH 2/3] Add fno-strict-overflow

[Raghavendra D Prabhu]

* On Mon, Jul 04, 2011 at 11:38:30PM +0100, Peter Maydell <address@hidden> 
wrote:
> On 4 July 2011 23:00, Raghavendra D Prabhu <address@hidden> wrote:
> > This is to avoid gcc optimizating out the comparison in assert,
> > due to assumption of signed overflow being undefined by default 
> > (-Werror=strict-overflow).
>
> > --- a/Makefile.hw
> > +++ b/Makefile.hw
> > @@ -9,7 +9,7 @@ include $(SRC_PATH)/rules.mak

> > $(call set-vpath, $(SRC_PATH):$(SRC_PATH)/hw)

> > -QEMU_CFLAGS+=-I.. -I$(SRC_PATH)/fpu
> > +QEMU_CFLAGS+=-I.. -I$(SRC_PATH)/fpu -fno-strict-overflow
>
> Can you give a more detailed description of the problem this is trying
> to solve? I think it would be nicer if we could remove the assumptions
> about signed overflows instead, if that's practical.
Following line in pcie.c:pcie_add_capability:505

    assert(offset < offset + size);

is what the compiler was warning about. The compiler optimizes out that
comparison without fno-strict-overflow flag. More information about it
is here -  http://www.airs.com/blog/archives/120 -- as already mentioned by 
Stefan.
> (Also, if we do want to add this compiler flag then it ought to be
> done in configure I think, as we do for -fno-strict-aliasing.)
Globally adding that flag can limits the optimizations of gcc since in
other places (loops) the undefined behavior can be advantageous, hence
added only to Makefile.hw.
> -- PMM

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Raghavendra Prabhu
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