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From: "meisenmann.lba@fh-salzburg.ac.at" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/60996] New: Bad code (I.e. needless insns) with option momit-leaf-frame-pointer; side-effect on non-leaf functions
Date: Tue, 29 Apr 2014 07:12:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60996

            Bug ID: 60996
           Summary: Bad code (I.e. needless insns) with option
                    momit-leaf-frame-pointer; side-effect on non-leaf
                    functions
           Product: gcc
           Version: 4.8.2
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: meisenmann.lba@fh-salzburg.ac.at

Using the option '-momit-leaf-frame-pointer' (with -fno-omit-frame-pointer) has
a side-effect on non-leaf functions. The code-snippets afterwards (produced
with an i386-elf cross-compiler for an IA32-target) will show the difference:

A) Without the option momit-leaf-frame-pointer:

Example1:
  pushl %ebp
  movl  %esp, %ebp
  pushl %ebx
  subl  $8, %esp
  movl  (%eax), %ebx
  pushl 12(%ebp)
  pushl 8(%ebp)
  pushl %ecx
  pushl %edx
  pushl %eax
  call  *12(%ebx)
  movl  -4(%ebp), %ebx
  leave
  ret

B) Option '-momit-leaf-frame-pointer' used:

Example1:
  pushl %ebp
  movl  %esp, %ebp
  pushl %ebx
  subl  $8, %esp
  movl  (%eax), %ebx
  pushl 12(%ebp)
  pushl 8(%ebp)
  pushl %ecx
  pushl %edx
  pushl %eax
  call  *12(%ebx)
  addl  $24, %esp
  movl  -4(%ebp), %ebx
  leave
  ret

In this case there'a an additional stack-adjustment (addl $24, %esp) before the
"final" stack-leave instruction. Some other examples with less forwarded
call-arguments have shown a few pop-insn instead of an 'add'.
These additional instruction(s) directly before reverting the stack-frame has
no functional effect, but will consume a few CPU-cycles.