https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105886
Bug ID: 105886
Summary: GCC optimizes structs with FP registers but ignores
alignment in this case
Product: gcc
Version: 11.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: keven.kloeckner at siemens dot com
Target Milestone: ---
Compiler output -v:
Using built-in specs.
COLLECT_GCC=aarch64-elf-gcc.exe
COLLECT_LTO_WRAPPER=c:/git/s74100gcc11/s74100cpu/s7p.tools/aarch64_gcc_elf_11.2.0/bin/../libexec/gcc/aarch64-elf/11.2.0/lto-wrapper.exe
Target: aarch64-elf
Configured with: ../../gcc-11.2.0//configure --host=x86_64-w64-mingw32
--build=x86_64-w64-mingw32
--prefix=/build/aarch64-elf_11.2.0-1/cross-gcc/aarch64-elf --target=aarch64-elf
--disable-nls --enable-multilib --with-multilib-list=lp64,ilp32
--enable-languages=c,c++ --disable-decimal-float
--with-sysroot=/build/aarch64-elf_11.2.0-1/cross-gcc/aarch64-elf
--without-headers --disable-shared --disable-threads --disable-lto
--disable-libmudflap --disable-libssp --disable-libgomp --disable-libffi
--disable-libstdcxx-pch --disable-win32-registry
--with-host-libstdcxx='-static-libgcc -Wl,-Bstatic,-lstdc++,-Bdynamic -lm'
--with-newlib --with-gcc --with-gnu-as --with-gnu-ld
--with-gmp=/build/aarch64-elf_11.2.0-1/host
--with-mpfr=/build/aarch64-elf_11.2.0-1/host
--with-mpc=/build/aarch64-elf_11.2.0-1/host
--with-isl=/build/aarch64-elf_11.2.0-1/host
--with-zstd=/build/aarch64-elf_11.2.0-1/host : (reconfigured)
../../gcc-11.2.0//configure --host=x86_64-w64-mingw32
--build=x86_64-w64-mingw32 --enable-languages=c,c++ --enable-multilib
--with-multilib-list=lp64,ilp32 --disable-lto --disable-libmudflap
--disable-libssp --disable-libgomp --disable-libffi --with-newlib --with-gcc
--with-gnu-ld --with-gnu-as --with-stabs --disable-shared --disable-threads
--disable-win32-registry --disable-nls --disable-libstdcxx-pch
--with-host-libstdcxx='-static-libgcc -Wl,-Bstatic,-lstdc++,-Bdynamic -lm'
--target=aarch64-elf --prefix=/build/aarch64-elf_11.2.0-1/cross-gcc/aarch64-elf
--with-gmp=/build/aarch64-elf_11.2.0-1/host
--with-mpfr=/build/aarch64-elf_11.2.0-1/host
--with-mpc=/build/aarch64-elf_11.2.0-1/host
--with-isl=/build/aarch64-elf_11.2.0-1/host
--with-zstd=/build/aarch64-elf_11.2.0-1/host
--with-sysroot=/build/aarch64-elf_11.2.0-1/cross-gcc/aarch64-elf
Thread model: single
Supported LTO compression algorithms: zlib zstd
gcc version 11.2.0 (GCC)
The problem occurs on our Windows to Arm Cortex A53 Cross compile.
No errors or warnings during compilation. We have enabled alignment check for
some purposes and we cannot disable it. This problem occurs also in C++, not
only in C (iam sorry if i put the bug wrong).
(for readability i removed all defines and link inputs)
Command Line:
../../../../aarch64_gcc_elf_11.2.0/bin/aarch64-elf-gcc -fno-builtin
-c -std=gnu11 -Wno-pointer-to-int-cast -Wno-int-to-pointer-cast
-Wno-maybe-uninitialized -Werror
-mabi=ilp32 -march=armv8-a+crypto+crc -mtune=cortex-a53 -mstrict-align
-mno-outline-atomics -Werror=return-type -Werror=format
-Werror=address -Werror=array-bounds -Werror=format-extra-args
-Werror=init-self -Wno-error=array-bounds -Wno-error=stringop-overflow
-Wno-error=free-nonheap-object -mlittle-endian -fno-common
-fno-merge-constants -ffunction-sections -fdata-sections
-fno-toplevel-reorder -O0 -Werror -nostdinc -ggdb
For some reason the GCC 11.2 is using floating point register instructions to
optimize initialization of structs if a lot of members are initialized with the
same constant successively.
The problem is that the GCC generates the following instructions:
stp q0, q0, [x0] //128 bit alignment required problem -> usually we use 64
bit
//alignment
str q0, [x0, #32] // 64 bit alignment -> ok
stur q0, [x0, #44] // offset with 32 bit alignment but intruction with 64 bit
//required alignment -> hard problem
My assumption is the compiler somehow thinks the FP registers are 4 byte or the
instructions require 4 byte alignment. I expected the compiler to ensure the
alignment if he is optimizing, which i can clearly see he is ignoring or doing
it wrong. If more informations are needed please contact me.
With kind regards
Keven Kloeckner
