03-11-2011
Bizzare optimization problem
I have a C program that, for the life of me, I can't see any possible stack corruption in but for some reason corrupts a local variable to 0 when not referenced. My compiler is gcc 4.3.4.
But my question's not really a code question, it's a compiler question. The glitch is weirdly specific: it happens at -O2 and higher. It does not happen when I individually and laboriously enable every possible -O1 and -O2 option(as reported by gcc -c -Q -O0 --help=optimizers, etc.) -- only the actual -O2 flag will cause it. What might gcc be doing in -O2 that it doesn't do when I enable every possible -O2 option?
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klcc(1) H. Peter Anvin klcc(1)
NAME
klcc - compile a program against klibc
SYNOPSIS
klcc [gcc options] [-o outfile] infile...
DESCRIPTION
klcc is a wrapper around gcc(1) and ld(1) which compiles and links a program against the klibc tiny C library. It supports most gcc
options.
Unlike gcc, klcc compiles with optimization on by default. Furthermore, the optimization level used depends on whether or not -g is speci-
fied, since klcc frequently uses options in the normal case which makes debugging impossible. Therefore, compile without -g, -O, -f or -m
option to use the default optimization level; this will generally result in the smallest binaries. You may want to use -s when linking,
however. Use -O0 to compile without any optimization whatsoever; this may not work depending on the version of gcc used.
-nostdinc
allows to turn off klibc include files.
Use the -shared or -static option to compile for and link against shared or static klibc. Note that shared klibc only supports running
against the exact same klibc binary as the binary was linked with.
In addition to standard gcc options, klcc supports options of the form -print-klibc-option, which prints the corresponding klibc configura-
tion option.
AUTHOR
Written by H. Peter Anvin <hpa@zytor.com>.
COPYRIGHT
Copyright (C) 2005 H. Peter Anvin - All Rights Reserved
Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the
"Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish,
distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to
the following conditions:
The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MER-
CHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
SEE ALSO
gcc(1)
klibc 1 March 2005 klcc(1)