https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118086
Bug ID: 118086
Summary: Missed Optimization: Redundant Copying of Large Struct
Parameter onto Stack
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 59891
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=59891&action=edit
Minimal test case.
When passing a large struct as an argument to a function, and the calling
function already has the large struct as a parameter, GCC redundantly copies
the struct parameter to the stack. I tested this with the (non-cross-compiling)
gcc, the aarch64 cross-compiler (aarch64-linux-gnu-gcc), and the riscv64
cross-compiler (riscv64-linux-gnu-gcc); with -O2, -O3, -Os, and -Oz. I have not
checked other CPU architectures.
A test case is in the attached file test.c, which I have reproduced below for
your convenience:
struct Small {
void *x;
};
struct Large {
void *x, *y, *z, *w;
};
extern int extern_func_small(struct Small);
extern int extern_func_large(struct Large);
int tail_call_small(struct Small x) {
return extern_func_small(x);
}
int non_tail_call_small(struct Small x) {
return ~extern_func_small(x);
}
int tail_call_large(struct Large x) {
return extern_func_large(x);
}
int non_tail_call_large(struct Large x) {
return ~extern_func_large(x);
}
x86_64 assembly, -O3:
tail_call_small:
.cfi_startproc
jmp extern_func_small@PLT
.cfi_endproc
non_tail_call_small:
.cfi_startproc
subq $8, %rsp
.cfi_def_cfa_offset 16
call extern_func_small@PLT
addq $8, %rsp
.cfi_def_cfa_offset 8
notl %eax
ret
.cfi_endproc
tail_call_large:
.cfi_startproc
jmp extern_func_large@PLT
.cfi_endproc
non_tail_call_large:
.cfi_startproc
subq $40, %rsp
.cfi_def_cfa_offset 48
movdqu 48(%rsp), %xmm0
movups %xmm0, (%rsp)
movdqu 64(%rsp), %xmm0
movups %xmm0, 16(%rsp)
call extern_func_large@PLT
addq $40, %rsp
.cfi_def_cfa_offset 8
notl %eax
ret
.cfi_endproc
aarch64 assembly, -O3:
tail_call_small:
.cfi_startproc
b extern_func_small
.cfi_endproc
non_tail_call_small:
.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_small
ldp x29, x30, [sp], 16
.cfi_restore 30
.cfi_restore 29
.cfi_def_cfa_offset 0
mvn w0, w0
ret
.cfi_endproc
tail_call_large:
.cfi_startproc
mov x2, x0
stp x29, x30, [sp, -48]!
.cfi_def_cfa_offset 48
.cfi_offset 29, -48
.cfi_offset 30, -40
mov x29, sp
add x1, sp, 16
ldp q30, q31, [x2]
mov x0, x1
str q30, [sp, 16]
str q31, [x1, 16]
bl extern_func_large
ldp x29, x30, [sp], 48
.cfi_restore 30
.cfi_restore 29
.cfi_def_cfa_offset 0
ret
.cfi_endproc
non_tail_call_large:
.cfi_startproc
mov x2, x0
stp x29, x30, [sp, -48]!
.cfi_def_cfa_offset 48
.cfi_offset 29, -48
.cfi_offset 30, -40
mov x29, sp
add x1, sp, 16
ldp q30, q31, [x2]
mov x0, x1
str q30, [sp, 16]
str q31, [x1, 16]
bl extern_func_large
mvn w0, w0
ldp x29, x30, [sp], 48
.cfi_restore 30
.cfi_restore 29
.cfi_def_cfa_offset 0
ret
.cfi_endproc
riscv64 assembly, -O3:
tail_call_small:
.cfi_startproc
tail extern_func_small@plt
.cfi_endproc
non_tail_call_small:
.cfi_startproc
addi sp,sp,-16
.cfi_def_cfa_offset 16
sd ra,8(sp)
.cfi_offset 1, -8
call extern_func_small@plt
ld ra,8(sp)
.cfi_restore 1
not a0,a0
sext.w a0,a0
addi sp,sp,16
.cfi_def_cfa_offset 0
jr ra
.cfi_endproc
tail_call_large:
.cfi_startproc
ld a2,0(a0)
ld a3,8(a0)
ld a4,16(a0)
ld a5,24(a0)
addi sp,sp,-48
.cfi_def_cfa_offset 48
mv a0,sp
sd ra,40(sp)
.cfi_offset 1, -8
sd a2,0(sp)
sd a3,8(sp)
sd a4,16(sp)
sd a5,24(sp)
call extern_func_large@plt
ld ra,40(sp)
.cfi_restore 1
addi sp,sp,48
.cfi_def_cfa_offset 0
jr ra
.cfi_endproc
non_tail_call_large:
.cfi_startproc
ld a2,0(a0)
ld a3,8(a0)
ld a4,16(a0)
ld a5,24(a0)
addi sp,sp,-48
.cfi_def_cfa_offset 48
mv a0,sp
sd ra,40(sp)
.cfi_offset 1, -8
sd a2,0(sp)
sd a3,8(sp)
sd a4,16(sp)
sd a5,24(sp)
call extern_func_large@plt
ld ra,40(sp)
.cfi_restore 1
not a0,a0
sext.w a0,a0
addi sp,sp,48
.cfi_def_cfa_offset 0
jr ra
.cfi_endproc
For a tail call with the "small" or pointer-sized struct, each of the
architectures seem to correctly elide the tail call into an unconditional
branch instruction. I have only a passing understanding of the calling
conventions for each of the architectures, so I do not know if the non-tail
call with the small struct is correctly optimized.
For a tail call with the "large" struct, only x86_64 correctly elides the tail
call into an unconditional branch instruction. The other two architectures
redundantly copy the struct parameter from the stack and onto the stack again
and pass the copy of the struct, rather than the original, as the argument to
extern_func. For the non-tail call with the "large" struct, all the
architectures likewise mistakenly copy the struct parameter from the stack and
onto the stack again and pass the copy of the struct, rather than the original,
to extern_func (I am aware that a non-tail call is not to be elided into an
unconditional branch instruction, but copying the struct to the stack and
passing instead the copy to extern_func is still unnecessary).
Again, I have only a passing understanding of the calling conventions for each
of the architectures in question, so I do not know whether or not, disregarding
the redundant struct copying, the amount that they subtract from the stack
pointer is optimal.
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
