Compile the following code with options -Os -mthumb -march=armv5te
union FloatIntUnion {
float fFloat;
int fSignBitInt;
};
static inline float fast_inc(float x) {
union FloatIntUnion data;
data.fFloat = x;
data.fSignBitInt += 1;
return data.fFloat;
}
extern int MyConvert(float);
extern float dumm();
int time_math() {
int i;
int sum = 0;
const int repeat = 100;
float f;
f = dumm();
for (i = repeat - 1; i >= 0; --i) {
sum += (int)f; f = fast_inc(f);
sum += (int)f; f = fast_inc(f);
sum += (int)f; f = fast_inc(f);
sum += (int)f; f = fast_inc(f);
}
f = dumm();
for (i = repeat - 1; i >= 0; --i) {
sum += MyConvert(f); f = fast_inc(f);
sum += MyConvert(f); f = fast_inc(f);
sum += MyConvert(f); f = fast_inc(f);
}
return sum;
}
Gcc generates:
push {r4, r5, r6, r7, lr}
sub sp, sp, #12
bl dumm
mov r4, #0
mov r6, #99
add r5, r0, #0
.L2:
add r0, r5, #0
bl __aeabi_f2iz
add r5, r5, #1
add r4, r0, r4
add r0, r5, #0
bl __aeabi_f2iz
add r5, r5, #1
add r4, r4, r0
add r0, r5, #0
bl __aeabi_f2iz
add r5, r5, #1
add r4, r4, r0
add r0, r5, #0
bl __aeabi_f2iz
add r5, r5, #1
add r4, r4, r0
sub r6, r6, #1
bcs .L2
bl dumm
mov r6, #99
add r5, r0, #0
.L3:
add r0, r5, #0
bl MyConvert
add r5, r5, #1
str r0, [sp, #4]
add r0, r5, #0
bl MyConvert
add r5, r5, #1
mov r7, r0
add r0, r5, #0
bl MyConvert
ldr r3, [sp, #4]
add r5, r5, #1
add r7, r7, r3
add r7, r7, r0
add r4, r4, r7
sub r6, r6, #1
bcs .L3
add sp, sp, #12
mov r0, r4
@ sp needed for prologue
pop {r4, r5, r6, r7, pc}
The source code contains 2 similar loops. But the generated code are quite
different. The code for first loop is as expected. After evaluating each
function, accumulates the returned value immediately. The code for second loop
is much worse. After evaluating each function, it saves the returned value to a
different place. After calling all functions in the same round of loop, it
accumulates all the saved results together. The code for second loop is larger
and slower, and even caused a register spilling.
The intermediate representation patterns for the two loops started to diverge
from pass float2int.c.078t.reassoc1. I don't know why gcc performs different
transforms on the two loops in this pass.
--
Summary: inefficient code to accumulate function return values
Product: gcc
Version: 4.5.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: carrot at google dot com
GCC build triplet: i686-linux
GCC host triplet: i686-linux
GCC target triplet: arm-eabi
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40783
