On Tue, Jul 5, 2011 at 4:36 PM, Raghavendra D Prabhu <raghu.prabhu13@xxxxxxxxx> wrote: > * On Mon, Jul 04, 2011 at 11:38:30PM +0100, Peter Maydell > <peter.maydell@xxxxxxxxxx> wrote: >> >> On 4 July 2011 23:00, Raghavendra D Prabhu <raghu.prabhu13@xxxxxxxxx> >> 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. Doing this on a per-subsystem or per-file basis does not make sense to me. This is a general C coding issue that needs to be settled for the entire codebase. We will not catch instances of overflow slipping in during patch review, so limiting the scope of -fno-strict-overflow is not feasible. I suggest we cover all of QEMU with -fwrapv instead of worrying about -fno-strict-overflow. That way we can get some optimizations and it reflects the model that we are all assuming: "This option instructs the compiler to assume that signed arithmetic overflow of addition, subtraction and multiplication wraps around using twos-complement representation. This flag enables some optimizations and disables others. This option is enabled by default for the Java front-end, as required by the Java language specification." http://gcc.gnu.org/onlinedocs/gcc-4.4.2/gcc/Code-Gen-Options.html Stefan -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
