Created attachment 25913 [details] "gcc -O2 -Wstrict-overflow" issues false alarm for this (I ran into this problem when building a test version of GNU Emacs.) "gcc -Wstrict-overflow -O2" sometimes issues a bogus warning when an integer cannot possibly overflow, because it is incremented in a location where it is known to not have the maximum integer value. Compile the attached file u.c with "gcc -Wstrict-overflow -O2 -S u.c". There will be a diagnostic: u.c: In function 'wait_reading_process_output': u.c:19:10: warning: assuming signed overflow does not occur when simplifying conditional to constant [-Wstrict-overflow] Signed overflow cannot possibly occur when "nfds++" is executed, for three independent reasons: 1. If nfds is INT_MAX, the line commented /*1*/ ensures that "nfds++" is unreachable. 2. If nfds is INT_MAX, the line commented /*2*/ ensures that "nfds++" is unreachable. 3. nfds starts at zero and is incremented in a loop body that is executed at most 1024 times, so it cannot possibly overflow. Each reason by itself is enough to guarantee no overflow in u.c. Mysteriously enough, changing the "1000" to "1" in line /*1*/ or line /*2*/ (or both) makes the bogus diagnostic vanish. For reference, here is the output of "gcc -v -save-temps -Wstrict-overflow -O2 -S u.c". Using built-in specs. COLLECT_GCC=gcc COLLECT_LTO_WRAPPER=/home/eggert/opt/Linux-x86_64/gcc-4.6.2/libexec/gcc/x86_64-unknown-linux-gnu/4.6.2/lto-wrapper Target: x86_64-unknown-linux-gnu Configured with: ../gcc-4.6.2/configure --prefix=/home/eggert/opt/Linux-x86_64/gcc-4.6.2 --disable-nls Thread model: posix gcc version 4.6.2 (GCC) COLLECT_GCC_OPTIONS='-v' '-save-temps' '-Wstrict-overflow' '-O2' '-S' '-mtune=generic' '-march=x86-64' /home/eggert/opt/Linux-x86_64/gcc-4.6.2/libexec/gcc/x86_64-unknown-linux-gnu/4.6.2/cc1 -E -quiet -v u.c -mtune=generic -march=x86-64 -Wstrict-overflow -O2 -fpch-preprocess -o u.i ignoring nonexistent directory "/home/eggert/opt/Linux-x86_64/gcc-4.6.2/lib/gcc/x86_64-unknown-linux-gnu/4.6.2/../../../../x86_64-unknown-linux-gnu/include" #include "..." search starts here: #include <...> search starts here: /home/eggert/opt/Linux-x86_64/gcc-4.6.2/lib/gcc/x86_64-unknown-linux-gnu/4.6.2/include /usr/local/include /home/eggert/opt/Linux-x86_64/gcc-4.6.2/include /home/eggert/opt/Linux-x86_64/gcc-4.6.2/lib/gcc/x86_64-unknown-linux-gnu/4.6.2/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-4.6.2/libexec/gcc/x86_64-unknown-linux-gnu/4.6.2/cc1 -fpreprocessed u.i -quiet -dumpbase u.c -mtune=generic -march=x86-64 -auxbase u -O2 -Wstrict-overflow -version -o u.s GNU C (GCC) version 4.6.2 (x86_64-unknown-linux-gnu) compiled by GNU C version 4.6.2, GMP version 4.3.2, MPFR version 3.0.0, MPC version 0.8.3-dev GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072 GNU C (GCC) version 4.6.2 (x86_64-unknown-linux-gnu) compiled by GNU C version 4.6.2, GMP version 4.3.2, MPFR version 3.0.0, MPC version 0.8.3-dev GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072 Compiler executable checksum: 26ba9716d225d191c012c3adc50d675b u.c: In function 'wait_reading_process_output': u.c:19:10: warning: assuming signed overflow does not occur when simplifying conditional to constant [-Wstrict-overflow] COMPILER_PATH=/home/eggert/opt/Linux-x86_64/gcc-4.6.2/libexec/gcc/x86_64-unknown-linux-gnu/4.6.2/:/home/eggert/opt/Linux-x86_64/gcc-4.6.2/libexec/gcc/x86_64-unknown-linux-gnu/4.6.2/:/home/eggert/opt/Linux-x86_64/gcc-4.6.2/libexec/gcc/x86_64-unknown-linux-gnu/:/home/eggert/opt/Linux-x86_64/gcc-4.6.2/lib/gcc/x86_64-unknown-linux-gnu/4.6.2/:/home/eggert/opt/Linux-x86_64/gcc-4.6.2/lib/gcc/x86_64-unknown-linux-gnu/ LIBRARY_PATH=/home/eggert/opt/Linux-x86_64/gcc-4.6.2/lib/gcc/x86_64-unknown-linux-gnu/4.6.2/:/home/eggert/opt/Linux-x86_64/gcc-4.6.2/lib/gcc/x86_64-unknown-linux-gnu/4.6.2/../../../../lib64/:/lib/../lib64/:/usr/lib/../lib64/:/home/eggert/opt/Linux-x86_64/gcc-4.6.2/lib/gcc/x86_64-unknown-linux-gnu/4.6.2/../../../:/lib/:/usr/lib/ COLLECT_GCC_OPTIONS='-v' '-save-temps' '-Wstrict-overflow' '-O2' '-S' '-mtune=generic' '-march=x86-64'
I can't seem to reproduce the warning; all of these are silent: $ /usr/local/bin/gcc -Wstrict-overflow -O2 -S u.c $ /usr/local/bin/gcc -Wstrict-overflow=1 -O2 -S u.c $ /usr/local/bin/gcc -Wstrict-overflow=2 -O2 -S u.c $ /usr/local/bin/gcc -Wstrict-overflow=3 -O2 -S u.c $ /usr/local/bin/gcc -Wstrict-overflow=4 -O2 -S u.c $ /usr/local/bin/gcc -Wstrict-overflow=5 -O2 -S u.c $ /usr/local/bin/gcc -Wstrict-overflow -O1 -S u.c $ /usr/local/bin/gcc -Wstrict-overflow -O0 -S u.c $ /usr/local/bin/gcc -Wstrict-overflow -Os -S u.c $ /usr/local/bin/gcc -Wstrict-overflow -Og -S u.c $ /usr/local/bin/gcc -Wstrict-overflow -O3 -S u.c $ /usr/local/bin/gcc -Wstrict-overflow -Ofast -S u.c And seeing as -Wstrict-overflow is being considered for deprecation in the next release of GCC anyways, I think I can close this.
(In reply to Eric Gallager from comment #1) > I can't seem to reproduce the warning Thanks, I can no longer reproduce the warning either. I used GCC 7.1.1 20170622 (Red Hat 7.1.1-3). So it looks like the problem is fixed.