Bug 2121683
| Summary: | __builtin_ia32_rdrand32_step() unconditionally emits cmov instruction for gcc -march=i586 compile | ||
|---|---|---|---|
| Product: | Red Hat Enterprise Linux 9 | Reporter: | Taketo Kabe <rkabe> |
| Component: | gcc | Assignee: | Marek Polacek <mpolacek> |
| gcc sub component: | gcc-toolset-11 | QA Contact: | qe-baseos-tools-bugs |
| Status: | CLOSED NOTABUG | Docs Contact: | |
| Severity: | medium | ||
| Priority: | unspecified | CC: | ahajkova, bstinson, fweimer, jakub, jwboyer, ohudlick, sipoyare |
| Version: | CentOS Stream | ||
| Target Milestone: | rc | ||
| Target Release: | --- | ||
| Hardware: | All | ||
| OS: | Unspecified | ||
| Whiteboard: | |||
| Fixed In Version: | Doc Type: | If docs needed, set a value | |
| Doc Text: | Story Points: | --- | |
| Clone Of: | Environment: | ||
| Last Closed: | 2022-08-26 12:50:46 UTC | Type: | Bug |
| Regression: | --- | Mount Type: | --- |
| Documentation: | --- | CRM: | |
| Verified Versions: | Category: | --- | |
| oVirt Team: | --- | RHEL 7.3 requirements from Atomic Host: | |
| Cloudforms Team: | --- | Target Upstream Version: | |
| Embargoed: | |||
Is there any CPU which does support RDRND ISA but not CMOV? /* For sane SSE instruction set generation we need fcomi instruction. It is safe to enable all CMOVE instructions. Also, RDRAND intrinsic expands to a sequence that includes conditional move. */ #define TARGET_CMOVE (TARGET_CMOV || TARGET_SSE || TARGET_RDRND) is the macro enabling the conditional move patterns. For RDRND this was done in https://gcc.gnu.org/r0-108854 and for SSE in https://gcc.gnu.org/r0-33571 Given that this seems to be intentional and that we don't support i586 anyway, I'm closing this BZ. Please feel free to open an upstream bug if you still think this is a bug. |
Description of problem: gcc builtin function __builtin_ia32_rdrand32_step() seems to emit cmov instruction unconditionally,even if -march=i586 (no cmov capability) is given to gcc. Version-Release number of selected component (if applicable): gcc version 11.3.1 20220421 (Red Hat 11.3.1-2) (GCC) How reproducible: always Steps to Reproduce: 0. install glibc-devel.i686 sudo dnf install glibc-devel.i686 1. create rdrand.c file as follows: /* gcc -mrdrnd -march=i586 -m32 rdrand.c */ #include <stdio.h> int main() { int s,r; s = __builtin_ia32_rdrand32_step(&r); printf("rdrand=%d status=%d\n", r, s); } 2. Compile with: gcc -mrdrnd -march=i586 -m32 rdrand.c 3. Examine the output by objdump -d: objdump -d a.out | grep -C 3 cmov Actual results: [kabe@centos-s-9 ~]$ objdump -d a.out | grep -C 3 cmov 80491a1: 0f c7 f0 rdrand %eax 80491a4: 89 02 mov %eax,(%edx) 80491a6: ba 01 00 00 00 mov $0x1,%edx 80491ab: 0f 42 c2 cmovb %edx,%eax 80491ae: 89 45 f4 mov %eax,-0xc(%ebp) 80491b1: 8b 45 f0 mov -0x10(%ebp),%eax 80491b4: 83 ec 04 sub $0x4,%esp Expected results: no match Additional info: gcc generates instructions on the fly for __builtin* functions. cmov seems to be generated by usage of gen_rtx_IF_THEN_ELSE() in rdrand_step: branch of gcc's gcc/config/i386/i386-expand.c, but I could not trace the cause why -march=i586 is not honoured by the on-the-fly compiler.