https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105342
Bug ID: 105342 Summary: [Extended Asm]Memory barrier geater than a function call Product: gcc Version: 12.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: middle-end Assignee: unassigned at gcc dot gnu.org Reporter: 570070308 at qq dot com Target Milestone: --- This is an enhancement request, not a bug. According to doc, using the "memory" clobber effectively forms a read/write memory barrier for the compiler. Through my tests, the memory barrier's range in Extended-Asm is even greater than a function call, it will barrier the memory in the function's own stack. I think it is useless and it may even generate more complex code with inline funtion. For example in the test case: ```test.c extern unsigned long int x[512]; void test(long int,long int,long int,long int,long int); void test1(long int); void test2(); int kkk() { unsigned long int k[512]; for (size_t i=0; i<512; ++i ) { k[i]=x[i]; } test1(k[0]); k[1]=3; k[2]=3; k[3]=3; k[4]=3; k[5]=3; test2(); test(k[1], k[2], k[3], k[4], k[5]); return 0; } ``` and ```test2.c void test2() { __asm__ volatile ("" : : :"memory" ); } ``` compile with ``` gcc-12 -fno-stack-protector -fcf-protection=none -fno-asynchronous-unwind-tables -mgeneral-regs-only -O3 -S test.c test2.c ``` than generate ```test.s: kkk: subq $4096, %rsp orq $0, (%rsp) subq $8, %rsp leaq x(%rip), %rsi movl $512, %ecx movq %rsp, %rdi rep movsq movq (%rsp), %rdi call test1@PLT xorl %eax, %eax call test2@PLT movl $3, %r8d movl $3, %ecx movl $3, %edx movl $3, %esi movl $3, %edi call test@PLT xorl %eax, %eax addq $4104, %rsp ret ``` ```test2.s test2: ret ``` The kkk's assembly code looks neat. However, if I put the contents of test2.c in test.c, then it will generate: ```test.s kkk: subq $4096, %rsp orq $0, (%rsp) subq $8, %rsp leaq x(%rip), %rsi movl $512, %ecx movq %rsp, %rdi rep movsq movq (%rsp), %rdi call test1@PLT movq $3, 8(%rsp) movq $3, 16(%rsp) movq $3, 24(%rsp) movq $3, 32(%rsp) movq $3, 40(%rsp) movq 40(%rsp), %r8 movq 32(%rsp), %rcx movq 24(%rsp), %rdx movq 16(%rsp), %rsi movq 8(%rsp), %rdi call test@PLT xorl %eax, %eax addq $4104, %rsp ret test2: ret ``` The compiler automatically inline the function test2() and think k[1], k[2], k[3], k[4], k[5] is barrier with the extended-asm, so the inlining test2() is even slower than not inlining it. The gcc-12 is installed by apt on ubuntu 22.04. Full compile log: ``` $ gcc-12 -fno-stack-protector -fcf-protection=none -fno-asynchronous-unwind-tables -mgeneral-regs-only -O3 -S test.c test2.c -v Using built-in specs. COLLECT_GCC=gcc-12 OFFLOAD_TARGET_NAMES=nvptx-none:amdgcn-amdhsa OFFLOAD_TARGET_DEFAULT=1 Target: x86_64-linux-gnu Configured with: ../src/configure -v --with-pkgversion='Ubuntu 12-20220319-1ubuntu1' --with-bugurl=file:///usr/share/doc/gcc-12/README.Bugs --enable-languages=c,ada,c++,go,d,fortran,objc,obj-c++,m2 --prefix=/usr --with-gcc-major-version-only --program-suffix=-12 --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 --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new --enable-gnu-unique-object --disable-vtable-verify --enable-plugin --enable-default-pie --with-system-zlib --enable-libphobos-checking=release --with-target-system-zlib=auto --enable-objc-gc=auto --enable-multiarch --disable-werror --enable-cet --with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic --enable-offload-targets=nvptx-none=/build/gcc-12-OcsLtf/gcc-12-12-20220319/debian/tmp-nvptx/usr,amdgcn-amdhsa=/build/gcc-12-OcsLtf/gcc-12-12-20220319/debian/tmp-gcn/usr --enable-offload-defaulted --without-cuda-driver --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu Thread model: posix Supported LTO compression algorithms: zlib zstd gcc version 12.0.1 20220319 (experimental) [master r12-7719-g8ca61ad148f] (Ubuntu 12-20220319-1ubuntu1) COLLECT_GCC_OPTIONS='-fno-stack-protector' '-fcf-protection=none' '-fno-asynchronous-unwind-tables' '-mgeneral-regs-only' '-O3' '-S' '-v' '-mtune=generic' '-march=x86-64' /usr/lib/gcc/x86_64-linux-gnu/12/cc1 -quiet -v -imultiarch x86_64-linux-gnu test.c -quiet -dumpbase test.c -dumpbase-ext .c -mgeneral-regs-only -mtune=generic -march=x86-64 -O3 -version -fno-stack-protector -fcf-protection=none -fno-asynchronous-unwind-tables -o test.s -Wformat -Wformat-security -fstack-clash-protection GNU C17 (Ubuntu 12-20220319-1ubuntu1) version 12.0.1 20220319 (experimental) [master r12-7719-g8ca61ad148f] (x86_64-linux-gnu) compiled by GNU C version 12.0.1 20220319 (experimental) [master r12-7719-g8ca61ad148f], GMP version 6.2.1, MPFR version 4.1.0, MPC version 1.2.1, isl version isl-0.24-GMP GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072 ignoring nonexistent directory "/usr/local/include/x86_64-linux-gnu" ignoring nonexistent directory "/usr/lib/gcc/x86_64-linux-gnu/12/include-fixed" ignoring nonexistent directory "/usr/lib/gcc/x86_64-linux-gnu/12/../../../../x86_64-linux-gnu/include" #include "..." search starts here: #include <...> search starts here: /usr/lib/gcc/x86_64-linux-gnu/12/include /usr/local/include /usr/include/x86_64-linux-gnu /usr/include End of search list. GNU C17 (Ubuntu 12-20220319-1ubuntu1) version 12.0.1 20220319 (experimental) [master r12-7719-g8ca61ad148f] (x86_64-linux-gnu) compiled by GNU C version 12.0.1 20220319 (experimental) [master r12-7719-g8ca61ad148f], GMP version 6.2.1, MPFR version 4.1.0, MPC version 1.2.1, isl version isl-0.24-GMP GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072 Compiler executable checksum: 200a3dd46f0674d1a8fcf2b133bc6014 COLLECT_GCC_OPTIONS='-fno-stack-protector' '-fcf-protection=none' '-fno-asynchronous-unwind-tables' '-mgeneral-regs-only' '-O3' '-S' '-v' '-mtune=generic' '-march=x86-64' /usr/lib/gcc/x86_64-linux-gnu/12/cc1 -quiet -v -imultiarch x86_64-linux-gnu test2.c -quiet -dumpbase test2.c -dumpbase-ext .c -mgeneral-regs-only -mtune=generic -march=x86-64 -O3 -version -fno-stack-protector -fcf-protection=none -fno-asynchronous-unwind-tables -o test2.s -Wformat -Wformat-security -fstack-clash-protection GNU C17 (Ubuntu 12-20220319-1ubuntu1) version 12.0.1 20220319 (experimental) [master r12-7719-g8ca61ad148f] (x86_64-linux-gnu) compiled by GNU C version 12.0.1 20220319 (experimental) [master r12-7719-g8ca61ad148f], GMP version 6.2.1, MPFR version 4.1.0, MPC version 1.2.1, isl version isl-0.24-GMP GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072 ignoring nonexistent directory "/usr/local/include/x86_64-linux-gnu" ignoring nonexistent directory "/usr/lib/gcc/x86_64-linux-gnu/12/include-fixed" ignoring nonexistent directory "/usr/lib/gcc/x86_64-linux-gnu/12/../../../../x86_64-linux-gnu/include" #include "..." search starts here: #include <...> search starts here: /usr/lib/gcc/x86_64-linux-gnu/12/include /usr/local/include /usr/include/x86_64-linux-gnu /usr/include End of search list. GNU C17 (Ubuntu 12-20220319-1ubuntu1) version 12.0.1 20220319 (experimental) [master r12-7719-g8ca61ad148f] (x86_64-linux-gnu) compiled by GNU C version 12.0.1 20220319 (experimental) [master r12-7719-g8ca61ad148f], GMP version 6.2.1, MPFR version 4.1.0, MPC version 1.2.1, isl version isl-0.24-GMP GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072 Compiler executable checksum: 200a3dd46f0674d1a8fcf2b133bc6014 COMPILER_PATH=/usr/lib/gcc/x86_64-linux-gnu/12/:/usr/lib/gcc/x86_64-linux-gnu/12/:/usr/lib/gcc/x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/12/:/usr/lib/gcc/x86_64-linux-gnu/ LIBRARY_PATH=/usr/lib/gcc/x86_64-linux-gnu/12/:/usr/lib/gcc/x86_64-linux-gnu/12/../../../x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/12/../../../../lib/:/lib/x86_64-linux-gnu/:/lib/../lib/:/usr/lib/x86_64-linux-gnu/:/usr/lib/../lib/:/usr/lib/gcc/x86_64-linux-gnu/12/../../../:/lib/:/usr/lib/ COLLECT_GCC_OPTIONS='-fno-stack-protector' '-fcf-protection=none' '-fno-asynchronous-unwind-tables' '-mgeneral-regs-only' '-O3' '-S' '-v' '-mtune=generic' '-march=x86-64' ```