On 9/23/24 13:51, Alexander Monakov wrote:
On Mon, 23 Sep 2024, Bradley Lucier via Gcc-help wrote:
So it appears that the Xeon X5460 CPU from 2008 doesn't have the rdtscp
instruction, which is used by default by this version of gcc on x86-64 when no
architecture is specified.
GCC doesn't emit neither the rdtsc nor rdtscp instruction on its own, it can
appear only if the source code is using the corresponding built-in or
instrinsic; and GCC doesn't "promote" rdtsc to rdtscp depending on -march.
So if you see rdtscp in binary there must be rdtscp in source code. Can you
investigate the source corresponding to the problematic code in the binary?
Thank you! The usage is
#ifdef ___CPU_x86
#if __has_builtin(__builtin_ia32_rdtscp)
#undef ___USE___builtin_ia32_rdtscp
#define ___USE___builtin_ia32_rdtscp
#endif
#endif
#ifdef ___DONT_USE___builtin_ia32_rdtscp
#undef ___USE___builtin_ia32_rdtscp
#endif
<cut>
#ifndef ___GET_CPUCYCLECOUNT
#ifdef ___USE___builtin_ia32_rdtscp
#ifdef __GNUC__
#if ___WORD_WIDTH == 64
#define ___GET_CPUCYCLECOUNT ___FIX(___s64_temp)
#define ___CPUCYCLECOUNTSTART \
__asm__ __volatile__ ("cpuid\n\trdtsc\n\tshl $32,%%rdx\n\tor
%%rdx,%%rax\n\tmov
%%rax,%0\n":"=r"(___s64_temp)::"%rax","%rbx","%rcx","%rdx");
#define ___CPUCYCLECOUNTEND \
__asm__ __volatile__ ("rdtscp\n\tshl $32,%%rdx\n\tor %%rdx,%%rax\n\tmov
%%rax,%0\n\tcpuid\n":"=r"(___s64_temp)::"%rax","%rbx","%rcx","%rdx");
#else
#define ___GET_CPUCYCLECOUNT ___FIX(___s32_temp)
#define ___CPUCYCLECOUNTSTART \
__asm__ __volatile__ ("cpuid\n\trdtsc\n\tmov
%%eax,%0\n":"=r"(___s32_temp)::"%eax","%ebx","%ecx","%edx");
#define ___CPUCYCLECOUNTEND \
__asm__ __volatile__ ("rdtscp\n\tmov
%%eax,%0\n\tcpuid\n":"=r"(___s32_temp)::"%eax","%ebx","%ecx","%edx");
#endif
#endif
#endif
#endif
So the use seems to be guarded by
__has_builtin(__builtin_ia32_rdtscp)
I compiled the code on the same machine that it failed on. So it thinks
it has the builtin and then it doesn't, right?
I get this output:
gcc-12 -Q --help=target | grep enabled
-m128bit-long-double [enabled]
-m64 [enabled]
-m80387 [enabled]
-malign-stringops [enabled]
-mdirect-extern-access [enabled]
-mfancy-math-387 [enabled]
-mfp-ret-in-387 [enabled]
-mfxsr [enabled]
-mglibc [enabled]
-mhard-float [enabled]
-mieee-fp [enabled]
-mlong-double-80 [enabled]
-mmmx [enabled]
-mno-sse4 [enabled]
-mpush-args [enabled]
-mred-zone [enabled]
-msse [enabled]
-msse2 [enabled]
-mstv [enabled]
-mtls-direct-seg-refs [enabled]
-mvzeroupper [enabled]
Is this relevant?
If I say
gcc -Q --help=target
I get
-march= x86-64
with a lot of other options enabled or disabled.
So, is there a way to tell what the default -march= value for a given version
of gcc is?
-Q --help=target gives that value.
OK, so that gave me "x86-64". Does that mean that code generated by
gcc-12 will run on any processor on this page:
https://gcc.gnu.org/onlinedocs/gcc-12.4.0/gcc/x86-Options.html
from "nocona" on?
Thanks.
Brad
