optimization/6722: global register hoisted out of loop containing a call

Sun May 19 13:46:00 GMT 2002

>Number:         6722
>Category:       optimization
>Synopsis:       global register hoisted out of loop containing a call
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          wrong-code
>Submitter-Id:   net
>Arrival-Date:   Sun May 19 13:46:01 PDT 2002
>Closed-Date:
>Last-Modified:
>Originator:     John Reiser
>Release:        gcc version 3.1
>Organization:
>Environment:
../gcc-3.1/configure --with-gcc-version-trigger=/home/gcc-3.1/gcc/version.c --host=i686-pc-linux-gnu --norecursion
>Description:
gcc -O2 [version 3.1 i686-pc-linux-gnu] hoists a "common
subexpression" involving a global register out of loop which
contains a call that modifies the global register.
This gives bad code because the expression is evaluated
only once, instead of every time around the loop
as required by C semantics.
>How-To-Repeat:
$ gcc -c -O2 bug.c
$ gdb bug.o
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This GDB was configured as "i386-redhat-linux"...(no debugging symbols found)...
(gdb) x/25i inflate_dynamic  
0xa0 <inflate_dynamic>: push   %ebp
0xa1 <inflate_dynamic+1>:       mov    %esp,%ebp
0xa3 <inflate_dynamic+3>:       push   %edi
0xa4 <inflate_dynamic+4>:       xor    %edi,%edi
0xa6 <inflate_dynamic+6>:       push   %esi
0xa7 <inflate_dynamic+7>:       sub    $0x10,%esp
0xaa <inflate_dynamic+10>:      mov    0x0,%eax
0xaf <inflate_dynamic+15>:      mov    0x0,%edx
0xb5 <inflate_dynamic+21>:      mov    %eax,0xfffffff4(%ebp)
0xb8 <inflate_dynamic+24>:      lea    0x7(%ebx),%esi  # bug.c:61   (k + n)
0xbb <inflate_dynamic+27>:      nop
0xbc <inflate_dynamic+28>:      lea    0x0(%esi,1),%esi
                # top of loop [here] omits computation of (k + n)
0xc0 <inflate_dynamic+32>:      mov    %ebx,%eax
0xc2 <inflate_dynamic+34>:      sar    $0x3,%eax
0xc5 <inflate_dynamic+37>:      mov    (%eax,%edx,1),%eax
0xc8 <inflate_dynamic+40>:      mov    %ebx,%ecx
0xca <inflate_dynamic+42>:      and    $0x7,%ecx
0xcd <inflate_dynamic+45>:      shr    %cl,%eax
0xcf <inflate_dynamic+47>:      mov    0x1c,%ecx
0xd5 <inflate_dynamic+53>:      and    %ecx,%eax
0xd7 <inflate_dynamic+55>:      test   %esi,%esi  # bug.c:61   if (0>=(k + n))
0xd9 <inflate_dynamic+57>:      jle    0xe8 <inflate_dynamic+72>
0xdb <inflate_dynamic+59>:      call   0xdc <inflate_dynamic+60>
0xe0 <inflate_dynamic+64>:      mov    0x0,%edx
0xe6 <inflate_dynamic+70>:      jmp    0xc0 <inflate_dynamic+32>  # goto bad top of loop
(gdb) q
>Fix:
Expressions involving a global register must not be moved
across procedure calls, unless it has been proved that the
procedure does not modify the global register.
>Release-Note:
>Audit-Trail:
>Unformatted:
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