http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50082
Bug #: 50082
Summary: -Wstrict-overflow mishandles typedef
Classification: Unclassified
Product: gcc
Version: 4.6.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: egg...@gnu.org
Created attachment 25011
--> http://gcc.gnu.org/bugzilla/attachment.cgi?id=25011
Sample program illustrating bogus warning
(I ran into this problem when building a test version of GNU Emacs.)
"gcc -Wstrict-overflow -O2" sometimes issues a bogus warning when an
int is declared via a typedef rather than using a bare 'int' keyword.
Compile the attached file r.c with "gcc -Wstrict-overflow -O2 -S r.c".
There will be a diagnostic:
r.c: In function 'insert_cache_boundary_bad':
r.c:19:6: warning: assuming signed overflow does not occur when reducing
constant in comparison [-Wstrict-overflow]
r.c has two functions that are nearly identical. The only difference
is that 'insert_cache_boundary_ok' uses plain 'int', whereas
'insert_cache_boundary_bad' uses 'foo' which is declared via "typedef
int foo;". The ..._ok function does not produce the bogus warning,
the ..._bad function does. (Both functions compile into the same
machine code.)
The diagnostic is bogus, since GCC can clearly deduce that signed
overflow is impossible here: 'i' must be nonnegative at the point of
the complaint, and 'i - 1' cannot possibly overflow.
For reference, here is the output of "gcc -v -save-temps
-Wstrict-overflow -O2 -S r.c".
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/home/eggert/opt/Linux-x86_64/gcc-4.6.1/libexec/gcc/x86_64-unknown-linux-gnu/4.6.1/lto-wrapper
Target: x86_64-unknown-linux-gnu
Configured with: ../gcc-4.6.1/configure
--prefix=/home/eggert/opt/Linux-x86_64/gcc-4.6.1 --disable-nls
Thread model: posix
gcc version 4.6.1 (GCC)
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-Wstrict-overflow' '-O2' '-S'
'-mtune=generic' '-march=x86-64'
/home/eggert/opt/Linux-x86_64/gcc-4.6.1/libexec/gcc/x86_64-unknown-linux-gnu/4.6.1/cc1
-E -quiet -v r.c -mtune=generic -march=x86-64 -Wstrict-overflow -O2
-fpch-preprocess -o r.i
ignoring nonexistent directory
"/home/eggert/opt/Linux-x86_64/gcc-4.6.1/lib/gcc/x86_64-unknown-linux-gnu/4.6.1/../../../../x86_64-unknown-linux-gnu/include"
#include "..." search starts here:
#include <...> search starts here:
/home/eggert/opt/Linux-x86_64/gcc-4.6.1/lib/gcc/x86_64-unknown-linux-gnu/4.6.1/include
/usr/local/include
/home/eggert/opt/Linux-x86_64/gcc-4.6.1/include
/home/eggert/opt/Linux-x86_64/gcc-4.6.1/lib/gcc/x86_64-unknown-linux-gnu/4.6.1/include-fixed
/usr/include
End of search list.
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-Wstrict-overflow' '-O2' '-S'
'-mtune=generic' '-march=x86-64'
/home/eggert/opt/Linux-x86_64/gcc-4.6.1/libexec/gcc/x86_64-unknown-linux-gnu/4.6.1/cc1
-fpreprocessed r.i -quiet -dumpbase r.c -mtune=generic -march=x86-64 -auxbase r
-O2 -Wstrict-overflow -version -o r.s
GNU C (GCC) version 4.6.1 (x86_64-unknown-linux-gnu)
compiled by GNU C version 4.6.1, GMP version 4.3.1, MPFR version 2.4.2, MPC
version 0.8.1
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
GNU C (GCC) version 4.6.1 (x86_64-unknown-linux-gnu)
compiled by GNU C version 4.6.1, GMP version 4.3.1, MPFR version 2.4.2, MPC
version 0.8.1
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: b0a4f47793c670c0e36c6193e4fd175b
r.c: In function 'insert_cache_boundary_bad':
r.c:19:6: warning: assuming signed overflow does not occur when reducing
constant in comparison [-Wstrict-overflow]
COMPILER_PATH=/home/eggert/opt/Linux-x86_64/gcc-4.6.1/libexec/gcc/x86_64-unknown-linux-gnu/4.6.1/:/home/eggert/opt/Linux-x86_64/gcc-4.6.1/libexec/gcc/x86_64-unknown-linux-gnu/4.6.1/:/home/eggert/opt/Linux-x86_64/gcc-4.6.1/libexec/gcc/x86_64-unknown-linux-gnu/:/home/eggert/opt/Linux-x86_64/gcc-4.6.1/lib/gcc/x86_64-unknown-linux-gnu/4.6.1/:/home/eggert/opt/Linux-x86_64/gcc-4.6.1/lib/gcc/x86_64-unknown-linux-gnu/
LIBRARY_PATH=/home/eggert/opt/Linux-x86_64/gcc-4.6.1/lib/gcc/x86_64-unknown-linux-gnu/4.6.1/:/home/eggert/opt/Linux-x86_64/gcc-4.6.1/lib/gcc/x86_64-unknown-linux-gnu/4.6.1/../../../../lib64/:/lib/../lib64/:/usr/lib/../lib64/:/home/eggert/opt/Linux-x86_64/gcc-4.6.1/lib/gcc/x86_64-unknown-linux-gnu/4.6.1/../../../:/lib/:/usr/lib/
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-Wstrict-overflow' '-O2' '-S'
'-mtune=generic' '-march=x86-64'
