| Summary: | Weird operation reordering when compiling with -O3/O2 leading to erroneous result | ||
|---|---|---|---|
| Product: | gcc | Reporter: | Cuero Bugot <cuerob> |
| Component: | c | Assignee: | Not yet assigned to anyone <unassigned> |
| Status: | RESOLVED DUPLICATE | ||
| Severity: | critical | CC: | alexcher, attardi, blake.tregre, cdfrey, chiabaut, cuerob, daniel, Dries.Decock, dvt, gcc-bugs, gcc2eran, hans.buchmann.wantuch, honza, kas, lucifer_ww, mihai.dontu, mueller, myan, rosenfeld, sb, schendel, sorenj, stillzhang, takahisa.yokota, tompa-l, unaiur |
| Priority: | P3 | ||
| Version: | 4.2.3 | ||
| Target Milestone: | --- | ||
| Host: | Target: | x86_64-linux-gnu | |
| Build: | Known to work: | ||
| Known to fail: | Last reconfirmed: | ||
2 instruction are inverted in the compiled code so the result is wrong. The compilation is ok when using no optimization, but the problem happen with -O3 and -O2. Here is the function to compile: void test2(uint32_t* source, uint32_t* dest) { *dest=*source; *(uint16_t*)dest = (*(uint16_t*)dest)<<1; } You could guess that the shift should happen after the copy. Here is what you get when dissassembling: 0000000000400500 <test2>: 400500: 66 d1 26 shlw (%rsi) 400503: 8b 07 mov (%rdi),%eax 400505: 89 06 mov %eax,(%rsi) 400507: c3 retq 400508: 0f 1f 84 00 00 00 00 nopl 0x0(%rax,%rax,1) 40050f: 00 Here is the information on my configuration: Using built-in specs. Target: x86_64-linux-gnu Configured with: ../src/configure -v --enable-languages=c,c++,fortran,objc,obj-c++,treelang --prefix=/usr --enable-shared --with-system-zlib --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --enable-nls --with-gxx-include-dir=/usr/include/c++/4.2 --program-suffix=-4.2 --enable-clocale=gnu --enable-libstdcxx-debug --enable-objc-gc --enable-mpfr --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu Thread model: posix gcc version 4.2.3 (Ubuntu 4.2.3-2ubuntu7) /usr/lib/gcc/x86_64-linux-gnu/4.2.3/cc1 -E -quiet -v test.c -mtune=generic -Wall -O2 -fpch-preprocess -o test.i ignoring nonexistent directory "/usr/local/include/x86_64-linux-gnu" ignoring nonexistent directory "/usr/lib/gcc/x86_64-linux-gnu/4.2.3/../../../../x86_64-linux-gnu/include" ignoring nonexistent directory "/usr/include/x86_64-linux-gnu" #include "..." search starts here: #include <...> search starts here: /usr/local/include /usr/lib/gcc/x86_64-linux-gnu/4.2.3/include /usr/include End of search list. /usr/lib/gcc/x86_64-linux-gnu/4.2.3/cc1 -fpreprocessed test.i -quiet -dumpbase test.c -mtune=generic -auxbase test -O2 -Wall -version -fstack-protector -fstack-protector -o test.s GNU C version 4.2.3 (Ubuntu 4.2.3-2ubuntu7) (x86_64-linux-gnu) compiled by GNU C version 4.2.3 (Ubuntu 4.2.3-2ubuntu7). GGC heuristics: --param ggc-min-expand=98 --param ggc-min-heapsize=128576 Compiler executable checksum: 04286fa30e0b1c736cc2bb914c15518c test.c: In function ‘main’: test.c:67: warning: implicit declaration of function ‘srand’ as --traditional-format -V -Qy -o test.o test.s GNU assembler version 2.18.0 (x86_64-linux-gnu) using BFD version (GNU Binutils for Ubuntu) 2.18.0.20080103 /usr/lib/gcc/x86_64-linux-gnu/4.2.3/collect2 --eh-frame-hdr -m elf_x86_64 --hash-style=both -dynamic-linker /lib64/ld-linux-x86-64.so.2 -o test /usr/lib/gcc/x86_64-linux-gnu/4.2.3/../../../../lib/crt1.o /usr/lib/gcc/x86_64-linux-gnu/4.2.3/../../../../lib/crti.o /usr/lib/gcc/x86_64-linux-gnu/4.2.3/crtbegin.o -L/usr/lib/gcc/x86_64-linux-gnu/4.2.3 -L/usr/lib/gcc/x86_64-linux-gnu/4.2.3 -L/usr/lib/gcc/x86_64-linux-gnu/4.2.3/../../../../lib -L/lib/../lib -L/usr/lib/../lib -L/usr/lib/gcc/x86_64-linux-gnu/4.2.3/../../.. test.o -lgcc --as-needed -lgcc_s --no-as-needed -lc -lgcc --as-needed -lgcc_s --no-as-needed /usr/lib/gcc/x86_64-linux-gnu/4.2.3/crtend.o /usr/lib/gcc/x86_64-linux-gnu/4.2.3/../../../../lib/crtn.o