Messages in this thread | | | Date | Tue, 19 Sep 2017 20:18:23 -0500 | From | Josh Poimboeuf <> | Subject | Re: [PATCH 2/2] x86/asm: Fix inline asm call constraints for clang |
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On Tue, Sep 19, 2017 at 01:45:28PM -0500, Josh Poimboeuf wrote: > For inline asm statements which have a CALL instruction, we list the > stack pointer as a constraint to convince GCC to ensure the frame > pointer is set up first: > > static inline void foo() > { > register void *__sp asm(_ASM_SP); > asm("call bar" : "+r" (__sp)) > } > > Unfortunately, that pattern causes clang to corrupt the stack pointer. > > There's actually an easier way to achieve the same goal in GCC, without > causing trouble for clang. If we declare the stack pointer register > variable as a global variable, and remove the constraint altogether, > that convinces GCC to always set up the frame pointer before inserting > *any* inline asm. > > It basically acts as if *every* inline asm statement has a CALL > instruction. It's a bit overkill, but the performance impact should be > negligible. > > Here are the vmlinux .text size differences with the following configs > on GCC: > > - defconfig > - defconfig without frame pointers > - Fedora distro config > - Fedora distro config without frame pointers > > defconfig defconfig-nofp distro distro-nofp > before 9796300 9466764 9076191 8789745 > after 9796941 9462859 9076381 8785325 > > With frame pointers, the text size increases slightly. Without frame > pointers, the text size decreases, and a little more significantly. > > Reported-by: Matthias Kaehlcke <mka@chromium.org> > Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
NAK - kbuild bot is reporting some cases where this patch doesn't force the frame pointer setup.
-- Josh
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