[Bug dynamic-link/31676] New: Configuring with CC="gcc -march=x86-64-v3" --with-rtld-early-cflags=-march=x86-64 results in linker failure

["fweimer at redhat dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=31676

            Bug ID: 31676
           Summary: Configuring with CC="gcc -march=x86-64-v3"
                    --with-rtld-early-cflags=-march=x86-64 results in
                    linker failure
           Product: glibc
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: dynamic-link
          Assignee: unassigned at sourceware dot org
          Reporter: fweimer at redhat dot com
  Target Milestone: ---

The error is:

gcc -march=x86-64-v3   -nostdlib -nostartfiles -shared -o …/build/elf/ld.so.new
              \
          -Wl,-z,relro -Wl,-z,defs      \
           \
          …/build/elf/librtld.os -Wl,--version-script=…/build/ld.map           
    \
          -Wl,-soname=ld-linux-x86-64.so.2
/usr/bin/ld: …/build/elf/librtld.os: in function `init_cpu_features':
…/git/elf/../sysdeps/x86/cpu-features.c:1202: undefined reference to
`_dl_runtime_resolve_fxsave'
/usr/bin/ld: …/build/elf/librtld.os: relocation R_X86_64_PC32 against undefined
hidden symbol `_dl_runtime_resolve_fxsave' can not be used when making a shared
object
/usr/bin/ld: final link failed: bad value
collect2: error: ld returned 1 exit status

The  reason is that we build with sysdeps/x86/dl-get-cpu-features.c with the
early CFLAGS (so -march=x86-64). This is necessary because the code runs very
early. But since

commit befe2d3c4dec8be2cdd01a47132e47bdb7020922
Author: H.J. Lu <hjl.tools@gmail.com>
Date:   Wed Feb 28 09:51:14 2024 -0800

    x86-64: Don't use SSE resolvers for ISA level 3 or above

    When glibc is built with ISA level 3 or above enabled, SSE resolvers
    aren't available and glibc fails to build:

    ld: .../elf/librtld.os: in function `init_cpu_features':
    .../elf/../sysdeps/x86/cpu-features.c:1200:(.text+0x1445f): undefined
reference to `_dl_runtime_resolve_fxsave'
    ld: .../elf/librtld.os: relocation R_X86_64_PC32 against undefined hidden
symbol `_dl_runtime_resolve_fxsave' can not be used when making a shared object
    /usr/local/bin/ld: final link failed: bad value

    For ISA level 3 or above, don't use _dl_runtime_resolve_fxsave nor
    _dl_tlsdesc_dynamic_fxsave.

    This fixes BZ #31429.
    Reviewed-by: Noah Goldstein <goldstein.w.n@gmail.com>

the referenced _dl_runtime_resolve_fxsave trampoline does not exist when the
rest of the dynamic linker is built for x86-64-v3.

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