https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110035
Bug ID: 110035
Summary: Missed optimization for dependent assignment
statements
Product: gcc
Version: 12.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: ptk.prasertsuk at gmail dot com
Target Milestone: ---
Created attachment 55212
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=55212&action=edit
Test case, compiled with -stdc++=20 -O2
The test case, when compiled, produces additional move instructions:
movdqu (%rdi), %xmm2
movdqu 16(%rdi), %xmm1
movdqu 32(%rdi), %xmm0
movl $48, %edi
movaps %xmm2, 32(%rsp)
movaps %xmm1, 16(%rsp)
movaps %xmm0, (%rsp)
call _Znwm@PLT
movdqa 32(%rsp), %xmm2
movdqa 16(%rsp), %xmm1
movdqa (%rsp), %xmm0
movq %rax, %rdi
movups %xmm2, (%rax)
movups %xmm1, 16(%rax)
movups %xmm0, 32(%rax)
compared to more optimized result using clang++ 14.0.0 with same flags:
callq _Znwm@PLT
movups (%rbx), %xmm0
movups 16(%rbx), %xmm1
movups 32(%rbx), %xmm2
movups %xmm0, (%rax)
movups %xmm1, 16(%rax)
movups %xmm2, 32(%rax)
movq %rax, %rdi
Clang has MemCpyOptPass which detects and removes memory dependency of the
second set of move instructions, which allows Dead Store Elimination pass to
remove the first set of move instructions.
g++-12 -v
Using built-in specs.
COLLECT_GCC=g++-12
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/12/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none:amdgcn-amdhsa
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu
12.1.0-2ubuntu1~22.04' --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-sZcx2y/gcc-12-12.1.0/debian/tmp-nvptx/usr,amdgcn-amdhsa=/build/gcc-12-sZcx2y/gcc-12-12.1.0/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 12.1.0 (Ubuntu 12.1.0-2ubuntu1~22.04)
