* On Tue, Jul 05, 2011 at 09:30:44PM +0100, Stefan Hajnoczi <stefanha@xxxxxxxxx> wrote:
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
I have removed that option from Makefile; instead replaced it with another
assert which shouldn't be affected by overflow.
=======================================
diff --git a/Makefile.hw b/Makefile.hw
index 23dac45..b9181ab 100644
--- 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$(SRC_PATH)/fpu -fno-strict-overflow
+QEMU_CFLAGS+=-I.. -I$(SRC_PATH)/fpu
include $(SRC_PATH)/Makefile.objs
diff --git a/hw/pcie.c b/hw/pcie.c
index 39607bf..cfb11fe 100644
--- a/hw/pcie.c
+++ b/hw/pcie.c
@@ -502,7 +502,7 @@ void pcie_add_capability(PCIDevice *dev,
uint16_t next;
assert(offset >= PCI_CONFIG_SPACE_SIZE);
- assert(offset < offset + size);
+ assert(UINT_MAX - size > offset);
assert(offset + size < PCIE_CONFIG_SPACE_SIZE);
assert(size >= 8);
assert(pci_is_express(dev));
--------------------------
Raghavendra Prabhu
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