Re: [lfs-support] Compiling gcc-4.9.2 from 7.7 without CMOV
On 2021-04-12 21:44 -0400, Michael Shell wrote: > On Fri, 9 Apr 2021 10:46:30 -0400 > Chris Fowler wrote: > > > I am fully aware that other OSS packages could use CMOV with inline > > assembly. I'm just wanting GCC to forget it even exists when it tries to > > implement CMOV during -03 optimization. > > > I think the most correct approach for your CPU is just to invoke > > -march=i586 > > to avoid the use of CMOV: > > https://clfs-support.cross-lfs.narkive.com/x1E4tLvU/i586-target-with-cmov You shouldn't need that. If you need this, it means your GCC is misconfigured. It may happen if you specify a wrong --with-arch value, or config.guess outputed i686-pc-linux-gnu (which is wrong on 586). I don't have an i586 so I can't test config.guess on it. You can try to run it. If it outputs i686, you can manually override it and report this as a bug, to config.guess maintainers (not GCC maintainers: config.guess is a seperated project at https://git.savannah.gnu.org/gitweb/?p=config.git). -- Xi Ruoyao School of Aerospace Science and Technology, Xidian University -- http://lists.linuxfromscratch.org/listinfo/lfs-support FAQ: http://www.linuxfromscratch.org/blfs/faq.html Unsubscribe: See the above information page Do not top post on this list. A: Because it messes up the order in which people normally read text. Q: Why is top-posting such a bad thing? A: Top-posting. Q: What is the most annoying thing in e-mail? http://en.wikipedia.org/wiki/Posting_style
Re: [lfs-support] Compiling gcc-4.9.2 from 7.7 without CMOV
On Fri, 9 Apr 2021 10:46:30 -0400 Chris Fowler wrote: > I am fully aware that other OSS packages could use CMOV with inline > assembly. I'm just wanting GCC to forget it even exists when it tries to > implement CMOV during -03 optimization. I think the most correct approach for your CPU is just to invoke -march=i586 to avoid the use of CMOV: https://clfs-support.cross-lfs.narkive.com/x1E4tLvU/i586-target-with-cmov This page might also be of help: https://stackoverflow.com/questions/60837883/removing-cmov-instructions-using-gcc-9-2-0-x86 which suggests adding the options (if you can't/don't use -march=i586) -fdisable-tree-phiopt[1-4] and -fdisable-rtl-ce[1-3] Note that there is a -mno-cmov for gcc, but that only seems to be available for the NDS32 platform: https://gcc.gnu.org/onlinedocs/gcc-6.3.0/gcc/Option-Summary.html Sigh. Cheers, Mike Shell -- http://lists.linuxfromscratch.org/listinfo/lfs-support FAQ: http://www.linuxfromscratch.org/blfs/faq.html Unsubscribe: See the above information page Do not top post on this list. A: Because it messes up the order in which people normally read text. Q: Why is top-posting such a bad thing? A: Top-posting. Q: What is the most annoying thing in e-mail? http://en.wikipedia.org/wiki/Posting_style
Re: [lfs-support] Compiling gcc-4.9.2 from 7.7 without CMOV
On 2021-04-09 10:46 -0400, Chris Fowler wrote: > I'm revisiting something I worked on a few years ago where I worked through > LFS > 7.7 on a i586 device that lacks CMOV. There are -march options for 'no MMX', > but nothing for 'no CMOV'. Where in the GCC source are those defined and can > I > create one for 'no CMOV' before I start building /tools > I am fully aware that other OSS packages could use CMOV with inline assembly. > I'm just wanting GCC to forget it even exists when it tries to implement CMOV > during -03 optimization. GCC won't generate CMOV if it can't make sure it's avaliable for the target. If you see some CMOV in your binaries, the possiblities are: 1. it comes from (maybe inline) assembly 2. some package have CMOV in (maybe inline) assembly, or it tells the compiler to generate CMOV, it means either: (1) it is buggy, or (2) it has runtime checking to avoid execution of CMOV on platforms w/o it 3. you are using wrong -march setting 4. there is a GCC bug (unlikely - I just took a look at GCC code for checking if CMOV is usable. The code seems correct.). -- Xi Ruoyao School of Aerospace Science and Technology, Xidian University -- http://lists.linuxfromscratch.org/listinfo/lfs-support FAQ: http://www.linuxfromscratch.org/blfs/faq.html Unsubscribe: See the above information page Do not top post on this list. A: Because it messes up the order in which people normally read text. Q: Why is top-posting such a bad thing? A: Top-posting. Q: What is the most annoying thing in e-mail? http://en.wikipedia.org/wiki/Posting_style
[lfs-support] Compiling gcc-4.9.2 from 7.7 without CMOV
I'm revisiting something I worked on a few years ago where I worked through LFS 7.7 on a i586 device that lacks CMOV. There are -march options for 'no MMX', but nothing for 'no CMOV'. Where in the GCC source are those defined and can I create one for 'no CMOV' before I start building /tools I am fully aware that other OSS packages could use CMOV with inline assembly. I'm just wanting GCC to forget it even exists when it tries to implement CMOV during -03 optimization. /proc/cpuinfo: processor : 0 vendor_id : Genuine RDC cpu family : 5 model : 8 model name : 05/08 stepping: 6 cpu MHz : 1000.136 fdiv_bug: no hlt_bug : no f00f_bug: no coma_bug: no fpu : yes fpu_exception : yes cpuid level : 1 wp : yes flags : fpu tsc cx8 mmx up bogomips: 2000.27 clflush size: 32 cache_alignment : 32 address sizes : 32 bits physical, 32 bits virtual -- http://lists.linuxfromscratch.org/listinfo/lfs-support FAQ: http://www.linuxfromscratch.org/blfs/faq.html Unsubscribe: See the above information page Do not top post on this list. A: Because it messes up the order in which people normally read text. Q: Why is top-posting such a bad thing? A: Top-posting. Q: What is the most annoying thing in e-mail? http://en.wikipedia.org/wiki/Posting_style
