https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108441
Bug ID: 108441
Summary: [12.2] Maybe missed optimization: loading an 16-bit
integer value from .rodata instead of an immediate
store
Product: gcc
Version: 12.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: 570070308 at qq dot com
Target Milestone: ---
For the C code:
```c
#include <stdint.h>
extern struct __attribute__((packed))
{
uint8_t size;
uint8_t pad;
uint16_t sec_num;
uint16_t offset;
uint16_t segment;
uint64_t sec_id;
} ldap;
//uint16_t x __attribute__((aligned(4096)));
void kkk()
{
ldap.size = 16;
ldap.pad = 0;
//x = 16;
}
```
gcc-12.2 compile it with O2, O3 or Ofast:
```
.globl kkk
.type kkk, @function
kkk:
movzwl .LC0(%rip), %eax
movw %ax, ldap(%rip)
ret
.size kkk, .-kkk
.section .rodata.cst2,"aM",@progbits,2
.align 2
.LC0:
.byte 16
.byte 0
```
It seems strange to load $16 to %eax from .rodata section. There is no such
problem with gcc-11.
gcc version:
```
Using built-in specs.
COLLECT_GCC=gcc
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 12.2.0-3ubuntu1'
--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-U8K4Qv/gcc-12-12.2.0/debian/tmp-nvptx/usr,amdgcn-amdhsa=/build/gcc-12-U8K4Qv/gcc-12-12.2.0/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 12.2.0 (Ubuntu 12.2.0-3ubuntu1)
```
The gcc is installed by apt on Ubuntu 22.10.
See more detail on:
https://stackoverflow.com/questions/75154687/is-this-a-missed-optimization-in-gcc-loading-an-16-bit-integer-value-from-roda
