On 02/04/2021 23:27, Michael Robins via Gcc-help wrote:
I am cross-compiling using "arm-none-eabi-gcc -mcpu=cortex-m0plus -O3"
for a target architecture that performs a multiply in a single cycle,
using gcc version 10.2.0 on a PC running Fedora Linux.
Is there an option to persuade the compiler to use the multiply
instruction automatically instead of shifts and adds when multiplying by
a constant?
In the example code below, gcc uses the trick of multiplying by a big
number instead of dividing by a small one (12 in this case). For my
target, the code from "-O3" is both longer and slower then that for "-Os".
foobar.c:
typedef struct {int x[3];} threeInts;
int foo(threeInts * p, threeInts * q)
{
return p - q;
}
#pragma GCC push_options
#pragma GCC optimize("-Os")
int bar(threeInts * p, threeInts * q)
{
return p - q;
}
#pragma GCC pop_options
foobar.s:
.cpu cortex-m0plus
.eabi_attribute 20, 1
.eabi_attribute 21, 1
.eabi_attribute 23, 3
.eabi_attribute 24, 1
.eabi_attribute 25, 1
.eabi_attribute 26, 1
.eabi_attribute 30, 2
.eabi_attribute 34, 0
.eabi_attribute 18, 4
.file "foobar.c"
.text
.align 1
.p2align 2,,3
.global foo
.arch armv6s-m
.syntax unified
.code 16
.thumb_func
.fpu softvfp
.type foo, %function
foo:
@ args = 0, pretend = 0, frame = 0
@ frame_needed = 0, uses_anonymous_args = 0
@ link register save eliminated.
subs r1, r0, r1
asrs r1, r1, #2
lsls r3, r1, #2
adds r3, r3, r1
lsls r0, r3, #4
adds r3, r3, r0
lsls r0, r3, #8
adds r3, r3, r0
lsls r0, r3, #16
adds r0, r3, r0
lsls r0, r0, #1
adds r0, r0, r1
@ sp needed
bx lr
.size foo, .-foo
.align 1
.global bar
.syntax unified
.code 16
.thumb_func
.fpu softvfp
.type bar, %function
bar:
@ args = 0, pretend = 0, frame = 0
@ frame_needed = 0, uses_anonymous_args = 0
@ link register save eliminated.
subs r0, r0, r1
ldr r1, .L4
asrs r0, r0, #2
muls r0, r1
@ sp needed
bx lr
.L5:
.align 2
.L4:
.word -1431655765
.size bar, .-bar
.ident "GCC: (GNU) 10.2.0"
Kind regards
Mike Robins
Could you raise a report in GCC bugzilla, with your testcase attached,
please?
R.
