[Bug libc/28771] New: %ebx optimization macros are incompatible with .altmacro in Systemtap probes

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

https://sourceware.org/bugzilla/show_bug.cgi?id=28771

            Bug ID: 28771
           Summary: %ebx optimization macros are incompatible with
                    .altmacro in Systemtap probes
           Product: glibc
           Version: 2.35
            Status: NEW
          Severity: normal
          Priority: P2
         Component: libc
          Assignee: unassigned at sourceware dot org
          Reporter: fweimer at redhat dot com
                CC: drepper.fsp at gmail dot com, hjl.tools at gmail dot com
  Target Milestone: ---
             Build: i686-linux-gnu
             Flags: security-

We need to build Fedora 36 with !CAN_USE_REGISTER_ASM_EBP because of
auto-vectorization at -O2. This results in a different build failure in
nptl/pthread_create.c because the probes there use .altmacro, which breaks
inline system calls later in this file.

To reproduce, build glibc with systemtap-sdt-devel-4.6-4.fc35 (or the Fedora
rawhide version) and:

../git/configure  --prefix=/usr CC="gcc -m32" CXX="g++ -m32"
--build=i686-linux-gnu libc_cv_can_use_register_asm_ebp=no --enable-systemtap

pthread_create.c: Assembler messages:
pthread_create.c:594: Error: % operator needs absolute expression
pthread_create.c:597: Error: % operator needs absolute expression
../sysdeps/unix/sysv/linux/default-sched.h:31: Error: % operator needs absolute
expression
../sysdeps/unix/sysv/linux/default-sched.h:34: Error: % operator needs absolute
expression
../sysdeps/unix/sysv/linux/default-sched.h:37: Error: % operator needs absolute
expression
../sysdeps/unix/sysv/linux/default-sched.h:40: Error: % operator needs absolute
expression
make[2]: *** [../o-iterator.mk:9:
/home/fweimer/src/gnu/glibc/build/nptl/pthread_create.o] Error 1

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