https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118098
Bug ID: 118098
Summary: Missed Optimization of memcpy into Unconditional
Branch
Product: gcc
Version: 14.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: jonathan.gruber.jg at gmail dot com
Target Milestone: ---
Created attachment 59902
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=59902&action=edit
Minimal test case.
GCC fails to optimize a memcpy into an unconditional branch.
Minimal test case (requires C23 for typeof):
/* Manually declare to avoid having to include a header. */
typedef typeof(sizeof(0)) size_t;
/* Manually declare to avoid having to include string.h. */
extern void *memcpy(void *restrict dst, const void *restrict src, size_t n);
void *fwd_memcpy(void *restrict dst, const void *restrict src, size_t n) {
/* Whole function body should optimize to:
* return memcpy(dst, src, n); */
/* memcpy returns dst. */
memcpy(dst, src, n);
return dst;
}
x86_64 assembly, with -O2 or -O3:
Expected:
fwd_memcpy:
.cfi_startproc
jmp memcpy@PLT
.cfi_endproc
Actual:
fwd_memcpy:
.cfi_startproc
subq $8, %rsp
.cfi_def_cfa_offset 16
call memcpy@PLT
addq $8, %rsp
.cfi_def_cfa_offset 8
ret
.cfi_endproc
aarch64 assembly, with -Oz, -Os, -O2, or -O3:
Expected:
fwd_memcpy:
.cfi_startproc
b memcpy
.cfi_endproc
Actual:
fwd_memcpy:
.cfi_startproc
stp x29, x30, [sp, -32]!
.cfi_def_cfa_offset 32
.cfi_offset 29, -32
.cfi_offset 30, -24
mov x29, sp
str x19, [sp, 16]
.cfi_offset 19, -16
mov x19, x0
bl memcpy
mov x0, x19
ldr x19, [sp, 16]
ldp x29, x30, [sp], 32
.cfi_restore 30
.cfi_restore 29
.cfi_restore 19
.cfi_def_cfa_offset 0
ret
.cfi_endproc
riscv64, with -Oz, -Os, -O2, or -O3:
Expected:
fwd_memcpy:
.cfi_startproc
tail memcpy@plt
.cfi_endproc
Actual:
fwd_memcpy:
.cfi_startproc
addi sp,sp,-16
.cfi_def_cfa_offset 16
sd s0,0(sp)
sd ra,8(sp)
.cfi_offset 8, -16
.cfi_offset 1, -8
mv s0,a0
call memcpy@plt
ld ra,8(sp)
.cfi_restore 1
mv a0,s0
ld s0,0(sp)
.cfi_restore 8
addi sp,sp,16
.cfi_def_cfa_offset 0
jr ra
.cfi_endproc
I did not test any other architectures, but I would not be surprised if they
also exhibit the issue.
Host system type: Arch Linux, x86_64.
gcc information:
Version: 14.2.1 20240910 (GCC)
Configured with: /build/gcc/src/gcc/configure
--enable-languages=ada,c,c++,d,fortran,go,lto,m2,objc,obj-c++,rust
--enable-bootstrap --prefix=/usr --libdir=/usr/lib --libexecdir=/usr/lib
--mandir=/usr/share/man --infodir=/usr/share/info
--with-bugurl=https://gitlab.archlinux.org/archlinux/packaging/packages/gcc/-/issues
--with-build-config=bootstrap-lto --with-linker-hash-style=gnu
--with-system-zlib --enable-__cxa_atexit --enable-cet=auto
--enable-checking=release --enable-clocale=gnu --enable-default-pie
--enable-default-ssp --enable-gnu-indirect-function --enable-gnu-unique-object
--enable-libstdcxx-backtrace --enable-link-serialization=1
--enable-linker-build-id --enable-lto --enable-multilib --enable-plugin
--enable-shared --enable-threads=posix --disable-libssp --disable-libstdcxx-pch
--disable-werror
aarch64-linux-gnu-gcc information:
Version: 14.2.0
Configured with: /build/aarch64-linux-gnu-gcc/src/gcc-14.2.0/configure
--prefix=/usr --program-prefix=aarch64-linux-gnu-
--with-local-prefix=/usr/aarch64-linux-gnu
--with-sysroot=/usr/aarch64-linux-gnu
--with-build-sysroot=/usr/aarch64-linux-gnu
--with-native-system-header-dir=/include --libdir=/usr/lib
--libexecdir=/usr/lib --target=aarch64-linux-gnu --host=x86_64-pc-linux-gnu
--build=x86_64-pc-linux-gnu --disable-nls --enable-default-pie
--enable-languages=c,c++,fortran --enable-shared --enable-threads=posix
--with-system-zlib --with-isl --enable-__cxa_atexit
--disable-libunwind-exceptions --enable-clocale=gnu --disable-libstdcxx-pch
--disable-libssp --enable-gnu-unique-object --enable-linker-build-id
--enable-lto --enable-plugin --enable-install-libiberty
--with-linker-hash-style=gnu --enable-gnu-indirect-function --disable-multilib
--disable-werror --enable-checking=release
riscv64-linux-gnu-gcc information:
Version: 14.2.0
Configured with: /build/riscv64-linux-gnu-gcc/src/gcc-14.2.0/configure
--prefix=/usr --program-prefix=riscv64-linux-gnu-
--with-local-prefix=/usr/riscv64-linux-gnu
--with-sysroot=/usr/riscv64-linux-gnu
--with-build-sysroot=/usr/riscv64-linux-gnu --libdir=/usr/lib
--libexecdir=/usr/lib --target=riscv64-linux-gnu --host=x86_64-pc-linux-gnu
--build=x86_64-pc-linux-gnu --with-system-zlib --with-isl
--with-linker-hash-style=gnu --disable-nls --disable-libunwind-exceptions
--disable-libstdcxx-pch --disable-libssp --disable-multilib --disable-werror
--enable-languages=c,c++ --enable-shared --enable-threads=posix
--enable-__cxa_atexit --enable-clocale=gnu --enable-gnu-unique-object
--enable-linker-build-id --enable-lto --enable-plugin
--enable-install-libiberty --enable-gnu-indirect-function --enable-default-pie
--enable-checking=release
