https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85244
Bug ID: 85244 Summary: Bad optimisation with flexible array member (may be related to -ftree-dominator-opts) Product: gcc Version: 8.0.1 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c Assignee: unassigned at gcc dot gnu.org Reporter: jean at pryen dot org Target Milestone: --- Created attachment 43863 --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=43863&action=edit Main program. The companion b.i is not provided but inlined Since gcc-6 (specific version bellow), there is a bad behavior with -O1 (and better) in the provided program that I tried to make it minimal. [A]. `gcc-8 -O1` cause the issue [B]. `gcc-6 -O1` also has the issue [C]. `gcc-5 -O1` was OK [D]. `gcc-8 -O1 -fno-tree-dominator-opts` does not have the issue When diffing the generated assembly code between good [D] and bad [A], it seems that the value `val.tab[idx]` is reused as is from a saved value of `val.tab[0]`, i.e. without considering the value of `idx`. --- bad/m.s 2018-04-05 21:07:11.602154334 -0400 +++ good/m.s 2018-04-05 21:07:11.542157334 -0400 @@ -13,11 +13,9 @@ main: .LFB11: .cfi_startproc - pushq %rbx + subq $8, %rsp .cfi_def_cfa_offset 16 - .cfi_offset 3, -16 - movl 12+val(%rip), %ebx - movl %ebx, %esi + movl 12+val(%rip), %esi leaq .LC0(%rip), %rdi movl $0, %eax call printf@PLT @@ -25,13 +23,14 @@ leaq .LC1(%rip), %rdi movl $0, %eax call printf@PLT - movl %ebx, %edx movq idx(%rip), %rsi + leaq val(%rip), %rax + movl 12(%rax,%rsi,4), %edx leaq .LC2(%rip), %rdi movl $0, %eax call printf@PLT movl $0, %eax - popq %rbx + addq $8, %rsp .cfi_def_cfa_offset 8 ret .cfi_endproc Note that the following changes in the source also cause the issue (and the diff in m.s) to disappear: * removing `const` from the `extern const struct s val` * removing or changing the types of `s.a` and `s.b` * defining the const `val` inside the source file --- Compilation line: gcc-8 -v ../m.c ../b.c -O1 -save-temps Output: Using built-in specs. COLLECT_GCC=gcc-8 COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/8/lto-wrapper OFFLOAD_TARGET_NAMES=nvptx-none OFFLOAD_TARGET_DEFAULT=1 Target: x86_64-linux-gnu Configured with: ../src/configure -v --with-pkgversion='Debian 8-20180321-1' --with-bugurl=file:///usr/share/doc/gcc-8/README.Bugs --enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c++ --prefix=/usr --with-gcc-major-version-only --program-suffix=-8 --program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new --enable-gnu-unique-object --disable-vtable-verify --enable-libmpx --enable-plugin --enable-default-pie --with-system-zlib --with-target-system-zlib --enable-objc-gc=auto --enable-multiarch --disable-werror --with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic --enable-offload-targets=nvptx-none --without-cuda-driver --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu Thread model: posix gcc version 8.0.1 20180321 (experimental) [trunk revision 258712] (Debian 8-20180321-1) COLLECT_GCC_OPTIONS='-v' '-O1' '-save-temps' '-mtune=generic' '-march=x86-64' /usr/lib/gcc/x86_64-linux-gnu/8/cc1 -E -quiet -v -imultiarch x86_64-linux-gnu ../m.c -mtune=generic -march=x86-64 -O1 -fpch-preprocess -o m.i ignoring nonexistent directory "/usr/local/include/x86_64-linux-gnu" ignoring nonexistent directory "/usr/lib/gcc/x86_64-linux-gnu/8/../../../../x86_64-linux-gnu/include" #include "..." search starts here: #include <...> search starts here: /usr/lib/gcc/x86_64-linux-gnu/8/include /usr/local/include /usr/lib/gcc/x86_64-linux-gnu/8/include-fixed /usr/include/x86_64-linux-gnu /usr/include End of search list. COLLECT_GCC_OPTIONS='-v' '-O1' '-save-temps' '-mtune=generic' '-march=x86-64' /usr/lib/gcc/x86_64-linux-gnu/8/cc1 -fpreprocessed m.i -quiet -dumpbase m.c -mtune=generic -march=x86-64 -auxbase m -O1 -version -o m.s GNU C17 (Debian 8-20180321-1) version 8.0.1 20180321 (experimental) [trunk revision 258712] (x86_64-linux-gnu) compiled by GNU C version 8.0.1 20180321 (experimental) [trunk revision 258712], GMP version 6.1.2, MPFR version 4.0.1, MPC version 1.1.0, isl version isl-0.19-GMP GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072 GNU C17 (Debian 8-20180321-1) version 8.0.1 20180321 (experimental) [trunk revision 258712] (x86_64-linux-gnu) compiled by GNU C version 8.0.1 20180321 (experimental) [trunk revision 258712], GMP version 6.1.2, MPFR version 4.0.1, MPC version 1.1.0, isl version isl-0.19-GMP GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072 Compiler executable checksum: 9ce40b2d6acfd4e504e10342a20ef051 COLLECT_GCC_OPTIONS='-v' '-O1' '-save-temps' '-mtune=generic' '-march=x86-64' as -v --64 -o m.o m.s Version de l'assembleur GNU 2.30 (x86_64-linux-gnu) utilisant la version BFD (GNU Binutils for Debian) 2.30 COLLECT_GCC_OPTIONS='-v' '-O1' '-save-temps' '-mtune=generic' '-march=x86-64' /usr/lib/gcc/x86_64-linux-gnu/8/cc1 -E -quiet -v -imultiarch x86_64-linux-gnu ../b.c -mtune=generic -march=x86-64 -O1 -fpch-preprocess -o b.i ignoring nonexistent directory "/usr/local/include/x86_64-linux-gnu" ignoring nonexistent directory "/usr/lib/gcc/x86_64-linux-gnu/8/../../../../x86_64-linux-gnu/include" #include "..." search starts here: #include <...> search starts here: /usr/lib/gcc/x86_64-linux-gnu/8/include /usr/local/include /usr/lib/gcc/x86_64-linux-gnu/8/include-fixed /usr/include/x86_64-linux-gnu /usr/include End of search list. COLLECT_GCC_OPTIONS='-v' '-O1' '-save-temps' '-mtune=generic' '-march=x86-64' /usr/lib/gcc/x86_64-linux-gnu/8/cc1 -fpreprocessed b.i -quiet -dumpbase b.c -mtune=generic -march=x86-64 -auxbase b -O1 -version -o b.s GNU C17 (Debian 8-20180321-1) version 8.0.1 20180321 (experimental) [trunk revision 258712] (x86_64-linux-gnu) compiled by GNU C version 8.0.1 20180321 (experimental) [trunk revision 258712], GMP version 6.1.2, MPFR version 4.0.1, MPC version 1.1.0, isl version isl-0.19-GMP GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072 GNU C17 (Debian 8-20180321-1) version 8.0.1 20180321 (experimental) [trunk revision 258712] (x86_64-linux-gnu) compiled by GNU C version 8.0.1 20180321 (experimental) [trunk revision 258712], GMP version 6.1.2, MPFR version 4.0.1, MPC version 1.1.0, isl version isl-0.19-GMP GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072 Compiler executable checksum: 9ce40b2d6acfd4e504e10342a20ef051 COLLECT_GCC_OPTIONS='-v' '-O1' '-save-temps' '-mtune=generic' '-march=x86-64' as -v --64 -o b.o b.s Version de l'assembleur GNU 2.30 (x86_64-linux-gnu) utilisant la version BFD (GNU Binutils for Debian) 2.30 COMPILER_PATH=/usr/lib/gcc/x86_64-linux-gnu/8/:/usr/lib/gcc/x86_64-linux-gnu/8/:/usr/lib/gcc/x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/8/:/usr/lib/gcc/x86_64-linux-gnu/ LIBRARY_PATH=/usr/lib/gcc/x86_64-linux-gnu/8/:/usr/lib/gcc/x86_64-linux-gnu/8/../../../x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/8/../../../../lib/:/lib/x86_64-linux-gnu/:/lib/../lib/:/usr/lib/x86_64-linux-gnu/:/usr/lib/../lib/:/usr/lib/gcc/x86_64-linux-gnu/8/../../../:/lib/:/usr/lib/ COLLECT_GCC_OPTIONS='-v' '-O1' '-save-temps' '-mtune=generic' '-march=x86-64' /usr/lib/gcc/x86_64-linux-gnu/8/collect2 -plugin /usr/lib/gcc/x86_64-linux-gnu/8/liblto_plugin.so -plugin-opt=/usr/lib/gcc/x86_64-linux-gnu/8/lto-wrapper -plugin-opt=-fresolution=m.res -plugin-opt=-pass-through=-lgcc -plugin-opt=-pass-through=-lgcc_s -plugin-opt=-pass-through=-lc -plugin-opt=-pass-through=-lgcc -plugin-opt=-pass-through=-lgcc_s --sysroot=/ --build-id --eh-frame-hdr -m elf_x86_64 --hash-style=gnu -dynamic-linker /lib64/ld-linux-x86-64.so.2 -pie /usr/lib/gcc/x86_64-linux-gnu/8/../../../x86_64-linux-gnu/Scrt1.o /usr/lib/gcc/x86_64-linux-gnu/8/../../../x86_64-linux-gnu/crti.o /usr/lib/gcc/x86_64-linux-gnu/8/crtbeginS.o -L/usr/lib/gcc/x86_64-linux-gnu/8 -L/usr/lib/gcc/x86_64-linux-gnu/8/../../../x86_64-linux-gnu -L/usr/lib/gcc/x86_64-linux-gnu/8/../../../../lib -L/lib/x86_64-linux-gnu -L/lib/../lib -L/usr/lib/x86_64-linux-gnu -L/usr/lib/../lib -L/usr/lib/gcc/x86_64-linux-gnu/8/../../.. m.o b.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/8/crtendS.o /usr/lib/gcc/x86_64-linux-gnu/8/../../../x86_64-linux-gnu/crtn.o COLLECT_GCC_OPTIONS='-v' '-O1' '-save-temps' '-mtune=generic' '-march=x86-64' What is displayed when executing the final program: ok val.tab[0] == 42 ok val.tab[1] == 1337 KO val.tab[1] == 42 What is expected: ok val.tab[0] == 42 ok val.tab[1] == 1337 KO val.tab[1] == 1337 Companion b.i file (only needed to build the executable and see the behavior) # 1 "b.c" # 1 "<built-in>" # 1 "<command-line>" # 31 "<command-line>" # 1 "/usr/include/stdc-predef.h" 1 3 4 # 32 "<command-line>" 2 # 1 "b.c" # 1 "t.h" 1 struct s { long a; int b; int tab[]; }; extern const struct s val; extern int idx; # 2 "b.c" 2 int idx = 1; const struct s val = { 0, 0, { 42, 1337 } };