[Bug target/78091] New: i386: Register allocation failure with -Os

["fw at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78091

            Bug ID: 78091
           Summary: i386: Register allocation failure with -Os
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: fw at gcc dot gnu.org
  Target Milestone: ---
            Target: i386

Created attachment 39874
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=39874&action=edit
posix_fallocate.i

The attached test case has been extracted from this glibc bug report:

  https://sourceware.org/bugzilla/show_bug.cgi?id=20729

We were under the impression that starting with GCC 5, we could issue
six-argument system calls (which need all registers except %esp) directly from
inline assembly.  Usually, this works, but it's clear that register allocation
is difficult under these circumstances.  With “-m32 -Os”, the test case fails
with:

posix_fallocate.i: In function ‘posix_fallocate’:
posix_fallocate.i:24:1: error: bp cannot be used in asm here
 }
 ^
posix_fallocate.i:24:1: error: bp cannot be used in asm here

This happens with:

gcc version 5.3.1 20160406 (Red Hat 5.3.1-6) (GCC) 
gcc version 7.0.0 20160518 (experimental) (GCC) 

If this cannot be fixed in GCC itself, we need a reliable compile-time test to
discover optimizer settings which cause this at configure time.