Messages in this thread | | | Date | Fri, 8 Mar 2019 10:56:58 +0000 | From | Russell King - ARM Linux admin <> | Subject | Re: [PATCH 2/2] ARM: futex: make futex_detect_cmpxchg more reliable |
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On Fri, Mar 08, 2019 at 11:16:47AM +0100, Ard Biesheuvel wrote: > Compiling the following code > > """ > #include <stdio.h> > > static void foo(void *a, int b) > { > asm("str %0, [%1]" :: "r"(a), "r"(b)); > } > > int main(void) > { > foo(NULL, 0); > } > """ > > with GCC 6.3 (at -O2) gives me > > .arch armv7-a > .eabi_attribute 28, 1 > .eabi_attribute 20, 1 > .eabi_attribute 21, 1 > .eabi_attribute 23, 3 > .eabi_attribute 24, 1 > .eabi_attribute 25, 1 > .eabi_attribute 26, 2 > .eabi_attribute 30, 2 > .eabi_attribute 34, 1 > .eabi_attribute 18, 4 > .file "futex.c" > .section .text.startup,"ax",%progbits > .align 1 > .p2align 2,,3 > .global main > .syntax unified > .thumb > .thumb_func > .fpu vfpv3-d16 > .type main, %function > main: > @ args = 0, pretend = 0, frame = 0 > @ frame_needed = 0, uses_anonymous_args = 0 > @ link register save eliminated. > movs r0, #0 > .syntax unified > @ 6 "/tmp/futex.c" 1 > str r0, [r0] > @ 0 "" 2 > .thumb > .syntax unified > bx lr > .size main, .-main > .ident "GCC: (Debian 6.3.0-18) 6.3.0 20170516" > .section .note.GNU-stack,"",%progbits > > and so GCC definitely behaves similar in this regard.
Let's take this further - a volatile is required for these cases to avoid gcc eliminating the asm() due to the output not being used:
#define NULL ((void *)0) static void foo(void *a, int b) { asm volatile("str %1, [%0]" : "=&r" (a) : "0" (a), "r" (b)); } int main(void) { foo(NULL, 0); }
produces:
mov r3, #0 mov r2, r3 str r2, [r2]
which looks to me to be incorrect to the GCC manual - the '&' on the output operand should mean that it does not conflict with other input operands, but clearly 'r2' has ended up being 'b' as well. I suspect this is a bug, or if not, is completely counter-intuitive from the description in the GCC manual.
Using "+r" (a) : "r" (b) also results in:
mov r3, #0 str r3, [r3]
It seems that only using "+&r" (a) : "r" (b) avoids a and b being in the same register, but I question whether we are stepping into undefined compiler behaviour with that.
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