[Bug target/63890] New: [4.9/5.0 regression] Compiling trivial program with -O -p leads to misaligned stack

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63890

            Bug ID: 63890
           Summary: [4.9/5.0 regression] Compiling trivial program with -O
                    -p leads to misaligned stack
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: fxcoudert at gcc dot gnu.org

Created attachment 33985
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=33985&action=edit
Assembler generated by trunk compiler with "-m32 -O -p"

Compile the following trivial program with trunk (rev. 217599) or the released
4.9.2 gcc leads to runtime segfault, on x86_64-apple-darwin14:

$ cat a.c
int main (void) { __builtin_exit (0); }
$ ./bin/gcc -m32 -O -p a.c && ./a.out
zsh: segmentation fault  ./a.out

This shows up as a failure of gcc.dg/20021014-1.c in the testsuite results
(https://gcc.gnu.org/ml/gcc-testresults/2014-11/msg01533.html).

Removing -p, or -O, or compiling for 64-bit doesn't trigger the bug. The
backtrace is:

  * frame #0: 0x9036f440 libdyld.dylib`misaligned_stack_error_
    frame #1: 0x00001f75 a.out`main + 17
    frame #2: 0x903716d9 libdyld.dylib`start + 1

Comparing the generated assembly by the trunk compiler (bad-trunk.s) with that
generated by the 4.8.3 compiler (ok-48.s) shows the difference is simple:

--- ok-48.s    2014-11-15 16:17:55.000000000 +0100
+++ bad-trunk.s    2014-11-15 16:17:44.000000000 +0100
@@ -1,4 +1,4 @@
-    .text
+    .section __TEXT,__text_startup,regular,pure_instructions
     .globl _main
 _main:
 LFB0:
@@ -7,12 +7,12 @@ LCFI0:
     movl    %esp, %ebp
 LCFI1:
     pushl    %ebx
-    subl    $20, %esp
+    subl    $16, %esp
 LCFI2:
     call    ___x86.get_pc_thunk.bx
 L1$pb:
     call mcount
-    movl    $0, (%esp)
+    pushl    $0
     call    _exit
 LFE0:
     .section __TEXT,__textcoal_nt,coalesced,pure_instructions