https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118091
Bug ID: 118091
Summary: Missed Optimization in Creating Medium-Sized Struct
and Passing It as Parameter
Product: gcc
Version: 14.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: jonathan.gruber.jg at gmail dot com
Target Milestone: ---
Created attachment 59893
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=59893&action=edit
Minimal test case.
GCC does not sufficiently optimize creating a medium-sized struct and passing
it as a parameter. I tested the architectures x86_64, aarch64, and riscv64, on
optimization levels -O2, -O3, -Os, and -Oz. The minimal test case is attached
to this bug report but reproduced below:
struct S {
void *x, *y;
};
extern int extern_func(struct S);
int tail_call(void *x, void *y) {
struct S s = { x, y };
return extern_func(s);
}
int non_tail_call(void *x, void *y) {
struct S s = { x, y };
return ~extern_func(s);
}
x86_64 assembly, -O3:
tail_call:
.cfi_startproc
xchgq %rdi, %rsi
movq %rsi, %rax
movq %rdi, %rsi
movq %rax, %rdi
jmp extern_func@PLT
.cfi_endproc
non_tail_call:
.cfi_startproc
xchgq %rdi, %rsi
subq $8, %rsp
.cfi_def_cfa_offset 16
movq %rsi, %rax
movq %rdi, %rsi
movq %rax, %rdi
call extern_func@PLT
addq $8, %rsp
.cfi_def_cfa_offset 8
notl %eax
ret
.cfi_endproc
aarch64 assembly, -O3:
tail_call:
.cfi_startproc
b extern_func
.cfi_endproc
non_tail_call:
.cfi_startproc
stp x29, x30, [sp, -16]!
.cfi_def_cfa_offset 16
.cfi_offset 29, -16
.cfi_offset 30, -8
mov x29, sp
bl extern_func
ldp x29, x30, [sp], 16
.cfi_restore 30
.cfi_restore 29
.cfi_def_cfa_offset 0
mvn w0, w0
ret
.cfi_endproc
riscv64 assembly, -O3:
tail_call:
.cfi_startproc
addi sp,sp,-16
.cfi_def_cfa_offset 16
addi sp,sp,16
.cfi_def_cfa_offset 0
tail extern_func@plt
.cfi_endproc
non_tail_call:
.cfi_startproc
addi sp,sp,-32
.cfi_def_cfa_offset 32
sd ra,24(sp)
.cfi_offset 1, -8
call extern_func@plt
ld ra,24(sp)
.cfi_restore 1
not a0,a0
sext.w a0,a0
addi sp,sp,32
.cfi_def_cfa_offset 0
jr ra
.cfi_endproc
I have only a passing familiarity with these architectures' calling
conventions, so please excuse any errors in my analysis below.
x86_64 effectively does a slightly slow no-op: it first swaps %rdi and %rsi and
then undoes that by swapping them again. The aarch64 version seems okay. The
tail call in riscv64 does another slightly slow no-op: it first decreases sp by
16 and then undoes that by increasing sp by 16. The non-tail call in riscv64
decreases sp by 32, which seems excessive when it only stores the 8-byte value
of ra on the stack.
I'm not sure if I filed this bug under the correct component
(rtl-optimization), so feel free to reassign it to the correct component after
the fact.
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
