[Bug lto/89896] New: -flto=4 is confused by presence of all.o in a local directory. -flto=1 is not.

[slyfox at inbox dot ru]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89896

            Bug ID: 89896
           Summary: -flto=4 is confused by presence of all.o in a local
                    directory. -flto=1 is not.
           Product: gcc
           Version: 8.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: lto
          Assignee: unassigned at gcc dot gnu.org
          Reporter: slyfox at inbox dot ru
                CC: marxin at gcc dot gnu.org
  Target Milestone: ---

The bug is originally discovered by Fabio Coatti in
https://bugs.gentoo.org/682018.

There isl-0.21 fails to link when built with  CFLAGS="-O2 -flto=4 -pipe"
LDFLAGS="${CFLAGS}". The trigger is all.c file name in isl source three. It
gets translated to all.o.

Here is the minimal reproducer:

  $ cat foo.bash
  #!/bin/bash

  export LANG=C

  cc=gcc

  mk() {
    local flag=$1

    rm -rf -- *.o *.so*

    ${cc} ${flag} -fPIC -c all.c -o all.o
    ${cc} ${flag} -fPIC -c isl.c -o isl.o
    # Note: all.o is not used here and yet in interferes with -flto=4
    ${cc} ${flag} -fPIC -shared isl.o -o libisl.so
  }

  # empty files are enough to trigger a bug
  touch all.c isl.c

  echo "=== -flto=1 (works) ==="
  mk "-flto=1"

  echo "=== -flto=4 (fails) ==="
  mk "-flto=4"

Running:

  $ ./foo.bash
  === -flto=1 (works) ===
  === -flto=4 (fails) ===
 
/usr/lib/gcc/x86_64-pc-linux-gnu/8.3.0/../../../../x86_64-pc-linux-gnu/bin/ld:
/usr/lib/gcc/x86_64-pc-linux-gnu/8.3.0/../../../../lib64/../lib64/Scrt1.o: in
function `_start':
/usr/src/debug/sys-libs/glibc-2.29-r1/glibc-2.29/csu/../sysdeps/x86_64/start.S:104:
undefined reference to `main'
  collect2: error: ld returned 1 exit status
  make: *** [<builtin>: all] Error 1
  lto-wrapper: fatal error: make returned 2 exit status
compilation terminated.
 
/usr/lib/gcc/x86_64-pc-linux-gnu/8.3.0/../../../../x86_64-pc-linux-gnu/bin/ld:
error: lto-wrapper failed
  collect2: error: ld returned 1 exit status

Note: all.o is never passed to gcc as an input. And yet it's presence
interferes with the driver.