On 11/14/2012 11:45 AM, Blower, Melanie wrote: > [1.] gcc -O0 assembly arch/x86/kvm/emulate.c gets compilation failure -- incorrect register restrictions > [2.] Full description of the problem/report: > I'm trying to compile this file at -O0, but gcc chokes in register allocation at the inline assembly. > > In the ordinary Linux build, this file compiles with gcc at -O2, without compilation errors. > > At -O0, gcc chokes with this message: > gcc -w -c ./emulateE.c // (using preprocessed file) > ./emulateE.c: In function `em_mul_ex': > ./emulateE.c:1918:5: error: can't find a register in class `AREG' while reloading `asm' > ./emulateE.c:1918:5: error: `asm' operand has impossible constraints > > Explanation: > The file contains an inline asm of a kind: > > __asm__ __volatile__ ( " ..... " : > > "=m" ((ctxt)->eflags), "=&r" (_tmp), "+a" (*rax), "+d" (*rdx), "+qm"(ex) : > "i" (11), "m" ((ctxt)->src . val), "a" (*rax), "d" (*rdx)); > > Note that "+a" in inputs already means that eax is the return value. An then "a" is used as an output constraint too. > Hi Melanie, Can you test the attached patch? -hpa
>From f8d8e2842ca05fd89788e35c087f02c6159b023a Mon Sep 17 00:00:00 2001 From: "H. Peter Anvin" <hpa@xxxxxxxxxxxxxxx> Date: Wed, 21 Nov 2012 13:29:20 -0800 Subject: [PATCH] x86, kvm: Remove incorrect redundant assembly constraint In __emulate_1op_rax_rdx, we use "+a" and "+d" which are input/output constraints, and *then* use "a" and "d" as input constraints. This is incorrect, but happens to work on some versions of gcc. However, it breaks gcc with -O0 and icc, and may break on future versions of gcc. Reported-by: Melanie Blower <melanie.blower@xxxxxxxxx> Signed-off-by: H. Peter Anvin <hpa@xxxxxxxxxxxxxxx> Link: http://lkml.kernel.org/r/B3584E72CFEBED439A3ECA9BCE67A4EF1B17AF90@xxxxxxxxxxxxxxxxxxxxxxxxxxxx --- arch/x86/kvm/emulate.c | 3 +-- 1 file changed, 1 insertion(+), 2 deletions(-) diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c index 39171cb..bba39bf 100644 --- a/arch/x86/kvm/emulate.c +++ b/arch/x86/kvm/emulate.c @@ -426,8 +426,7 @@ static void invalidate_registers(struct x86_emulate_ctxt *ctxt) _ASM_EXTABLE(1b, 3b) \ : "=m" ((ctxt)->eflags), "=&r" (_tmp), \ "+a" (*rax), "+d" (*rdx), "+qm"(_ex) \ - : "i" (EFLAGS_MASK), "m" ((ctxt)->src.val), \ - "a" (*rax), "d" (*rdx)); \ + : "i" (EFLAGS_MASK), "m" ((ctxt)->src.val)); \ } while (0) /* instruction has only one source operand, destination is implicit (e.g. mul, div, imul, idiv) */ -- 1.7.11.7
