http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50012
Summary: C++ front end misses -Wsign-compare warnings when
extraneous parentheses are present
Product: gcc
Version: 4.7.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: mi...@it.uu.se
In this test case two similar functions compare an unsigned int with a signed
int, differing only in whether the unsigned sub-expression has an extra pair of
parentheses around it or not. The C FE correctly issues a -Wsign-compare
diagnostic for both functions, but the C++ FE fails to do so for the function
with the extra parentheses.
> cat bug.c
int foo(unsigned int *a, int b)
{
return (*a) <= b;
}
int bar(unsigned int *a, int b)
{
return *a <= b;
}
> gcc/xgcc -Bgcc/ -O2 -Wsign-compare -S bug.c
bug.c: In function 'foo':
bug.c:3:17: warning: comparison between signed and unsigned integer expressions
[-Wsign-compare]
bug.c: In function 'bar':
bug.c:8:15: warning: comparison between signed and unsigned integer expressions
[-Wsign-compare]
> gcc/g++ -Bgcc/ -O2 -Wsign-compare -S /tmp/bug.c
/tmp/bug.c: In function 'int bar(unsigned int*, int)':
/tmp/bug.c:8:18: warning: comparison between signed and unsigned integer
expressions [-Wsign-compare]
This difference exists in 4.7-20110806, 4.6.1, and 4.5.3, but not in 4.4.6.
The test case is derived from ipa-inline-analysis.c, specifically Jan Hubicka's
recent change in r177484. That change introduced a signed/unsigned comparison,
but due to this C++ FE bug and the recent "build stage2 etc with g++ by
default" change, the error went undetected. That is, until people started
using --disable-build-poststage1-with-cxx to reduce bootstrap times, where it
ultimately breaks bootstrap, see PR50005.
