https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105519
Bug ID: 105519
Summary: Unnecessary memcpy() copy for empty asm volatile
Product: gcc
Version: 11.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: alexgpg at gmail dot com
Target Milestone: ---
Created attachment 52937
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=52937&action=edit
Source code preprocessed file
Use asm volatile("" : "+m,r"(value) : : "memory") with specific constraints
("+m,r") with large objects(array or structs with size > 8192) cause full copy
for object and that copy stay unused.
Maybe it's an issue in the register allocator. The compiler uses "r" instead of
"m" even for large objects.
The following code cause caused unnecessary copying with memcpy()
```
struct Large {
int arr[2049];
};
extern Large obj;
namespace {
template <class Tp>
inline __attribute__((always_inline)) void DoNotOptimize(Tp& value) {
asm volatile("" : "+m,r"(value) : : "memory");
}
}
void foo() {
DoNotOptimize(obj);
}
```
Generate assembly code
g++ -Wall -Wextra -O3 -save-temps -fno-stack-protector -S do_not_optimize.cpp
Generated assembly code (x86_64)
```
foo():
subq $8216, %rsp # 1. Extend stack size
movl $8196, %edx # 2. %edx = 8196 = sizeof(Large) = sizeof(int) * 2049.
# Prepare 3d arg (n - size) for memcpy()
leaq obj(%rip), %rsi # 3. %rsi = &obj. Prepare 2d arg (src) for memcpy().
movq %rsp, %rdi # 4. %rdi = %rsp. %rdi points to the top of the stack.
# Prepace 1-st arg (dest) for memcpy().
call memcpy@PLT # 5. Call memcpy(dest=%rdi, src=%rsi ,n=%edx=8196)
addq $8216, %rsp # 6. Reduce stack size
ret # 7. Return from function
```
What code do?
1. Extent stack size (line 1)
2. Copy data to new extended space in stack (line 2-7)
3. Reduce stack size back (line 6)
Looks like copy is not needed.
Notes
* -fno-stack-protector used just for small assembly code. Enable stack
protection doesn't change behavior
* gcc generates memcpy() call only if size of object > 8192
* https://godbolt.org/z/hPYfcrqbW - Godbolt Compiler Explorer Playground
Versions: 11.2.0, but looks like versions 4.1.2 - 11.12(all available on
godbolt) is also affected
# gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-pc-linux-gnu/11.2.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: /build/gcc/src/gcc/configure
--enable-languages=c,c++,ada,fortran,go,lto,objc,obj-c++,d --enable-bootstrap
--prefix=/usr --libdir=
/usr/lib --libexecdir=/usr/lib --mandir=/usr/share/man
--infodir=/usr/share/info --with-bugurl=https://bugs.archlinux.org/
--with-linker-hash-style
=gnu --with-system-zlib --enable-__cxa_atexit --enable-cet=auto
--enable-checking=release --enable-clocale=gnu --enable-default-pie
--enable-defaul
t-ssp --enable-gnu-indirect-function --enable-gnu-unique-object
--enable-linker-build-id --enable-lto --enable-multilib --enable-plugin
--enable-sh
ared --enable-threads=posix --disable-libssp --disable-libstdcxx-pch
--disable-werror --with-build-config=bootstrap-lto --enable-link-serialization
=1 gdc_include_dir=/usr/include/dlang/gdc
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 11.2.0 (GCC)
