On Fri, 17 Sept 2021 at 09:29, Xi Ruoyao wrote: > > On Thu, 2021-09-16 at 22:46 -0400, NightStrike via Gcc-help wrote: > > On Tue, Sep 7, 2021 at 3:57 AM Jonathan Wakely via Gcc-help > > <gcc-help@xxxxxxxxxxx> wrote: > > > > > > On Tue, 7 Sep 2021, 08:03 unlvsur unlvsur via Gcc-help, < > > > gcc-help@xxxxxxxxxxx> wrote: > > > > > > > I try to cross compile to another slower machine. -march=native > > > > works on > > > > that architectures, but I would like to know what is the value of > > > > -march=??? For -march=native. Is there a way to print march value > > > > out?? > > > > > > > > > > It doesn't choose a single value. It enables all the individual > > > options > > > like -msse and that combination of options might not correspond to any > > > particular -march value. > > > > Is that true? I mean, in principle, I know I've reported a bug where > > -march=native scanned /proc/cpuinfo (or however it got the info) and > > came up with a different result than -march=k8 (or whatever I was > > reporting at the time), but that was a bug that some helpful people > > fixed. If gcc doesn't have an -march for a particular esoteric arch, > > then fine, but if it does, I'd think that this would be a bug similar > > to what I experienced previously. > > It's documented clearly: > > native > This selects the CPU to generate code for at compilation time > by determining the processor type of the compiling machine. > Using -march=native enables all instruction subsets supported > by the local machine (hence the result might not run on > different machines). Using -mtune=native produces code > optimized for the local machine under the constraints of the > selected instruction set. > > However if it produce something can't run on your machine, > this is a bug. Yes, but that's not what we're discussing. The question is whether -march=native always corresponds to one of the other -march=xxx options or not. On my machine if I do -march=native the verbose asm shows: -march=skylake -mmmx -mpopcnt -msse -msse2 -msse3 -mssse3 -msse4.1 -msse4.2 -mavx -mavx2 -mno-sse4a -mno-fma4 -mno-xop -mfma -mno-avx512f -mbmi -mbmi2 -maes -mpclmul -mno-avx512vl -mno-avx512bw -mno-avx512dq -mno-avx512cd -mno-avx512er -mno-avx512pf -mno-avx512vbmi -mno-avx512ifma -mno-avx5124vnniw -mno-avx5124fmaps -mno-avx512vpopcntdq -mno-avx512vbmi2 -mno-gfni -mno-vpclmulqdq -mno-avx512vnni -mno-avx512bitalg -mno-avx512bf16 -mno-avx512vp2intersect -mno-3dnow -madx -mabm -mno-cldemote -mclflushopt -mno-clwb -mno-clzero -mcx16 -mno-enqcmd -mf16c -mfsgsbase -mfxsr -mhle -msahf -mno-lwp -mlzcnt -mmovbe -mno-movdir64b -mno-movdiri -mno-mwaitx -mno-pconfig -mno-pku -mno-prefetchwt1 -mprfchw -mno-ptwrite -mno-rdpid -mrdrnd -mrdseed -mrtm -mno-serialize -msgx -mno-sha -mno-shstk -mno-tbm -mno-tsxldtrk -mno-vaes -mno-waitpkg -mno-wbnoinvd -mxsave -mxsavec -mxsaveopt -mxsaves -mno-amx-tile -mno-amx-int8 -mno-amx-bf16 -mno-uintr -mno-hreset -mno-kl -mno-widekl -mno-avxvnni -mno-avx512fp16 --param=l1-cache-size=32 --param=l1-cache-line-size=64 --param=l2-cache-size=8192 -mtune=skylake So the question is whether that is any different to what is enabled by just -march=skylake alone.The answer is yes, it's different. On my machine -march=native adds -mabm and -mrtm which are not enabled by -march=skylake, and it also uses different L1 and L2 cache sizes. Compare: $ diff <(g++ -march=skylake -Q --help=target --help=params) <(g++ -march=native -Q --help=target --help=params) --- /dev/fd/63 2021-09-17 11:23:41.462736840 +0100 +++ /dev/fd/62 2021-09-17 11:23:41.463736847 +0100 @@ -9,7 +9,7 @@ -m8bit-idiv [disabled] -m96bit-long-double [disabled] -mabi= sysv - -mabm [disabled] + -mabm [enabled] -maccumulate-outgoing-args [disabled] -maddress-mode= long -madx [enabled] @@ -149,7 +149,7 @@ -mred-zone [enabled] -mregparm= 6 -mrtd [disabled] - -mrtm [disabled] + -mrtm [enabled] -msahf [enabled] -mserialize [disabled] -msgx [enabled] @@ -328,8 +328,8 @@ --param=jump-table-max-growth-ratio-for-size= 300 --param=jump-table-max-growth-ratio-for-speed= 800 --param=l1-cache-line-size= 64 - --param=l1-cache-size= 64 - --param=l2-cache-size= 512 + --param=l1-cache-size= 32 + --param=l2-cache-size= 8192 --param=large-function-growth= 100 --param=large-function-insns= 2700 --param=large-stack-frame-growth= 1000 I get the same results for i7-8650U and i7-6700K. Is that a bug in the -march=skylake settings? Maybe, I don't know. But it shows that -march=native is not the same as just the named -march option for my machines.
