A simple function to store a small struct ends up writing the struct to
memory twice unnecessarily (plus writing it for real the third time).
The function's args are passed in r0-r3 (with r1-r3 being the struct).
It compiles to a sequence containing the following loads and stores,
among others:
[1] stmfd sp!, {r0, r1, r2, r4, r5}
[2] stmia r0, {r1, r2, r3}
[3] ldmia r0, {r0, r1, r2}
[4] stmia r3, {r0, r1, r2}
[5] ldmfd sp!, {r1, r2, r3, r4, r5}
[2] and [3] seem the most egregious. (r0 points to stack space.)
However, [1] and [5] are odd too; I don't know why it's taking the time
to preserve r1-r3. Line [4] is the line that's actually necessary.
Adjusting the optimization level can eliminate the extra saves in [1],[5],
but all optimization levels seem to perform the extra save in [2],[3].
Environment:
System: OpenBSD underhill.hhhh.org 4.4 UNDERHILL#1 i386
host: i386-unknown-openbsd4.4
build: i386-unknown-openbsd4.4
target: arm-none-elf
configured with: ../gcc-4.4.1/configure --with-system-zlib --disable-nls
--target=arm-none-elf --prefix=/usr/local/cross/arm-elf --enable-languages=c
--with-cpu=arm7tdmi --with-newlib --with-gmp=/usr/local --with-mpfr=/usr/local
--with-gnu-as --with-gnu-ld --disable-libssp
--enable-version-specific-runtime-libs
How-To-Repeat:
$ arm-none-elf-gcc -mcpu=arm7tdmi -mthumb-interwork -Os -Wa,-alhd -c
rstore.c
struct queue {
unsigned short used;
struct queue_entry {
unsigned short aux;
void (*func)(int);
int arg;
} entries[16];
};
void enqueue(struct queue *q, struct queue_entry ent)
{
q->entries[q->used++] = ent;
}
--
Summary: [ARM] Suboptimal code generated to store a struct
Product: gcc
Version: 4.4.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: wiml at hhhh dot org
GCC build triplet: i386-unknown-openbsd4.4
GCC host triplet: i386-unknown-openbsd4.4
GCC target triplet: arm-none-elf
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41021
