On Tue, May 21, 2019 at 9:46 AM Uros Bizjak <ubiz...@gmail.com> wrote:
>
> On Tue, May 21, 2019 at 12:20 AM tedheadster <tedheads...@gmail.com> wrote:
> >
> > On Mon, May 20, 2019 at 2:57 PM Uros Bizjak <ubiz...@gmail.com> wrote:
> > >
> > > On Mon, May 20, 2019 at 6:12 PM tedheadster <tedheads...@gmail.com> wrote:
> > > > Did you instead mean "zeroing %EBX and %ECX regs should be enough"?
> > >
> > > Ah, yes. This is what I meant to say. The patch clears %ebx and %ecx.
> > >
> >
> > Uros,
> > your patch worked on real 32-bit hardware. The assembly output is
> > nearly identical to mine, with merely a re-ordering when setting the
> > %eax, %ebx, and %ecx registers.
>
> Attached patch fixes the core of the problem. We can change __cpuid
> itself to use zeroing, unless we can prove that we use constant
> argument, different than 1. __cpuid is mostly used with constant
> argument, so constant propagation does its job. As an example:
>
> --cut here--
> #include "cpuid.h"
>
> int main ()
> {
> unsigned int eax, ebx, ecx, edx;
>
> if (!__get_cpuid (1, &eax, &ebx, &ecx, &edx))
> __builtin_abort ();
>
> printf ("%#x, %#x, %#x, %#x\n", eax, ebx, ecx, edx);
> return 0;
> }
> --cut here--
>
> results in:
>
> 2f: 31 f6 xor %esi,%esi
> 31: 89 f0 mov %esi,%eax
> 33: 0f a2 cpuid
> 35: 85 c0 test %eax,%eax
> 37: 0f 84 fc ff ff ff je 39 <main+0x39>
> 3d: 83 ec 0c sub $0xc,%esp
> 40: 89 f3 mov %esi,%ebx
> 42: 89 f1 mov %esi,%ecx
> 44: b8 01 00 00 00 mov $0x1,%eax
> 49: 0f a2 cpuid
2019-05-21 Uroš Bizjak <ubiz...@gmail.com>
* config/i386/cpuid.h (__cpuid): For 32bit targets, zero
%ebx and %ecx bafore calling cpuid with leaf 1 or
non-constant leaf argument.
Bootstrapped and regression tested on x86_64-linux-gnu {,-m32}.
Committed to mainline SVN, will be backported to all active branches.
Uros.
