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From: werner@almesberger.net To: gcc-gnats@gcc.gnu.org Subject: debug/10005: gcc -O -g does not produce location information for aliases of a variable Date: Sat, 08 Mar 2003 14:36:00 -0000 [thread overview] Message-ID: <200303081429.h28ETwm27010@almesberger.net> (raw) >Number: 10005 >Category: debug >Synopsis: gcc -O -g does not produce location information for aliases of a variable >Confidential: no >Severity: serious >Priority: medium >Responsible: unassigned >State: open >Class: sw-bug >Submitter-Id: net >Arrival-Date: Sat Mar 08 14:36:00 UTC 2003 >Closed-Date: >Last-Modified: >Originator: Werner Almesberger >Release: 3.3 20030303 (prerelease) >Organization: >Environment: System: Linux ar 2.4.18 #5 Mon Mar 18 09:25:24 ART 2002 i686 unknown Architecture: i686 host: i686-pc-linux-gnu build: i686-pc-linux-gnu target: i686-pc-linux-gnu configured with: ./configure --prefix=/usr/local/gcc20030303/ --enable-languages=c >Description: gcc -O -g does not produce location information for aliases of a variable, so a debugger may fail to change the variable. Of course, one could argue that with -O, all bets are off as far as the accuracy of debugging information is concerned, but this still seems to be a fairly bad case. >How-To-Repeat: Example (with Red Hat's gcc 3.1 20011127 on ia32; gcc 3.3 yields different code, but the problem remains): $ cat <<EOF >foo.c int bar; int main(int argc) { bar = (int) &argc; bar = argc; label: return argc; } EOF $ gcc -O -g foo.c $ readelf -w a.out ... DW_AT_name : (indirect string, offset: 0x3a): argc ... DW_AT_location : 2 byte block: 91 8 (DW_OP_fbreg: 8; ) ... DW_AT_name : (indirect string, offset: 0x55): label ... DW_AT_low_pc : 0x80483a9 134513577 ... $ objdump -d a.out ... 08048398 <main>: ... 80483a1: 8b 45 08 mov 0x8(%ebp),%eax 80483a4: a3 24 95 04 08 mov %eax,0x8049524 80483a9: c9 leave ... So suppose we place a breakpoint at "label" (0x80483a9), change "argc" (supposedly at 8(%ebp)), and let the program continue. In this case, "main" will still return the original value, and ignore the change. >Fix: >Release-Note: >Audit-Trail: >Unformatted:
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