https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104683
Bug ID: 104683
Summary: -march=haswell generates invalid instructions on
Celeron Haswell CPUs
Product: gcc
Version: 10.3.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: other
Assignee: unassigned at gcc dot gnu.org
Reporter: pc-gcc22 at crowcastle dot net
Target Milestone: ---
Many of the -march options enable instruction types like AVX that are not
available on Pentium and Celeron processors in that family.
I encountered this on a Haswell Celeron system running gcc 10.3.1. I suspect
this is the same on many other versions.
gcc --version
gcc (Gentoo 10.3.1_p20211126 p0) 10.3.1 20211126
diff <(gcc -march=haswell -Q --help=target) <(gcc -march=native -Q
--help=target)
12c12
< -mabm [disabled]
---
> -mabm [enabled]
26,27c26,27
< -mavx [enabled]
< -mavx2 [enabled]
---
> -mavx [disabled]
> -mavx2 [disabled]
48,49c48,49
< -mbmi [enabled]
< -mbmi2 [enabled]
---
> -mbmi [disabled]
> -mbmi2 [disabled]
65c65
< -mf16c [enabled]
---
> -mf16c [disabled]
70c70
< -mfma [enabled]
---
> -mfma [disabled]
186c186
< -mxsave [enabled]
---
> -mxsave [disabled]
188c188
< -mxsaveopt [enabled]
---
> -mxsaveopt [disabled]
Fortunately, -march=native is fine, but I encountered a project that detects
the processor family and was setting -march=haswell, which failed with illegal
instructions.
While I would argue that -march=[family] should compile only using instructions
available for all processors in [family], I could also see an argument for this
being a documentation bug where the definition for -march=[family] should warn
that it will enable features only available on some CPUs in that family, which
is not what many users would expect. (There might be cause for separate -march
definitions for lower-end processors, but I don't have brilliant ideas for
syntax, and that might be just adding too much complexity.)
