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optimization/6810: GCC 3.1 generates incorrect code if -O2 -mcpu=pentium3 -fomit-frame-pointer are specified

From: vacb4 at freenet dot de
To: gcc-gnats at gcc dot gnu dot org
Date: 25 May 2002 04:31:37 -0000
Subject: optimization/6810: GCC 3.1 generates incorrect code if -O2 -mcpu=pentium3 -fomit-frame-pointer are specified
Reply-to: vacb4 at freenet dot de


>Number:         6810
>Category:       optimization
>Synopsis:       GCC 3.1 generates incorrect code if -O2 -mcpu=pentium3 -fomit-frame-pointer are specified
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          wrong-code
>Submitter-Id:   net
>Arrival-Date:   Fri May 24 21:36:00 PDT 2002
>Closed-Date:
>Last-Modified:
>Originator:     Peter Joachim
>Release:        gcc version 3.1
>Organization:
>Environment:
Windows NT 4.0 SP6, Pentium III (Coppermine) 850 MHz, Cygwin
>Description:
Compiling the code sample using -O2 -march=pentium3 -fomit-frame-pointer generates code that does not correctly calculate the addresses of local variables.
Namely the address of local array c1 in testfunc2 is not correctly passed to dummy_func.
The reason is the leal instruction (*) in the prologue to testfunc2 that incorrectly appears before _alloca is called, but should be placed after the _alloca call.
_testfunc2:
        pushl   %ebp
        movl    $24076, %eax
        pushl   %edi
        pushl   %esi
        pushl   %ebx
        leal    18064(%esp), %esi (*)
        call    __alloca
        movl    %esi, 8(%esp)
>How-To-Repeat:
Correct code is generated with -fno-omit-frame-pointer:
$ gcc -Wall -O2 -march=pentium3 -fno-omit-frame-pointer -fverbose-asm bugofp.i -o bugofp.exe
$ ./bugofp.exe
a1a1a1a1 a2a2a2a2 a3a3a3a3 a4a4a4a4 a5a5a5a5 a6a6a6a6 a7a7a7a7 a8a8a8a8 a9a9a9a9 aaaaaaaa abababab acacacac

-fomit-frame-pointer triggers the bug:
$ gcc -Wall -O2 -march=pentium3 -fomit-frame-pointer -fverbose-asm bugofp.i -o bugofp.exe
$ ./bugofp.exe
a1a1a1a1 a2a2a2a2 0 a4a4a4a4 a5a5a5a5 a6a6a6a6 a7a7a7a7 a8a8a8a8 a9a9a9a9 aaaaaaaa abababab acacacac
>Fix:
Don't specify -march=pentium3 -fomit-frame-pointer
>Release-Note:
>Audit-Trail:
>Unformatted:
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