https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118092
Bug ID: 118092
Summary: Missed Optimization of Variadic Functions
Product: gcc
Version: 14.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: jonathan.gruber.jg at gmail dot com
Target Milestone: ---
GCC insufficiently optimizes variadic functions. I tested this with
optimization levels -Oz, -Os, -O2, and -O3; on architectures x86_64, aarch64,
and riscv64. Attached are the architecture-specific preprocessed versions of
this minimal test case:
#include <stdarg.h>
void yes_ap(int dummy, ...) {
va_list ap;
va_start(ap, dummy);
va_end(ap);
}
void no_ap(int dummy, ...) {
/* Nothing. */
}
x86_64 assembly, -O3:
yes_ap:
.cfi_startproc
subq $40, %rsp
.cfi_def_cfa_offset 48
movq %fs:40, %rax
movq %rax, 24(%rsp)
leaq 48(%rsp), %rax
movq %rax, 8(%rsp)
movq 24(%rsp), %rax
subq %fs:40, %rax
jne .L5
addq $40, %rsp
.cfi_remember_state
.cfi_def_cfa_offset 8
ret
.L5:
.cfi_restore_state
call __stack_chk_fail@PLT
.cfi_endproc
no_ap:
.cfi_startproc
ret
.cfi_endproc
aarch64 assembly, -O3:
yes_ap:
.cfi_startproc
sub sp, sp, #32
.cfi_def_cfa_offset 32
add x0, sp, 32
stp x0, x0, [sp]
str x0, [sp, 16]
stp wzr, wzr, [sp, 24]
add sp, sp, 32
.cfi_def_cfa_offset 0
ret
.cfi_endproc
no_ap:
.cfi_startproc
ret
.cfi_endproc
riscv64 assembly, -O3:
yes_ap:
.cfi_startproc
addi sp,sp,-80
.cfi_def_cfa_offset 80
addi a0,sp,24
sd a1,24(sp)
sd a2,32(sp)
sd a3,40(sp)
sd a4,48(sp)
sd a5,56(sp)
sd a6,64(sp)
sd a7,72(sp)
sd a0,8(sp)
addi sp,sp,80
.cfi_def_cfa_offset 0
jr ra
.cfi_endproc
no_ap:
.cfi_startproc
addi sp,sp,-64
.cfi_def_cfa_offset 64
sd a1,8(sp)
sd a2,16(sp)
sd a3,24(sp)
sd a4,32(sp)
sd a5,40(sp)
sd a6,48(sp)
sd a7,56(sp)
addi sp,sp,64
.cfi_def_cfa_offset 0
jr ra
.cfi_endproc
x86_64 and aarch64 seem to sufficiently optimize no_ap to just a return
instruction; riscv64 redundantly saves argument registers a1-a7 to the stack.
None of the three architectures sufficiently optimizes yes_ap.
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
