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

[Raghavendra D Prabhu <raghu.prabhu13@gmail.com>]

* On Mon, Jul 04, 2011 at 11:38:30PM +0100, Peter Maydell <peter.maydell@linaro.org> wrote:
>On 4 July 2011 23:00, Raghavendra D Prabhu <raghu.prabhu13@gmail.com> 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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