https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64774
Bug ID: 64774
Summary: [ARM/thumb] missed optimization: pc relative ldr used
when constant can be derived from register
Product: gcc
Version: 4.9.3
Status: UNCONFIRMED
Severity: minor
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: bruck.michael at googlemail dot com
In the example the second write address of each function can either be
generated using 'str' with an immediate offset or using an adds with an
immediate addend.
The compiler does this for test1-3. For test4-6 it emits a pc-relative ldr
instruction and a 32bit constant. This increases size and likely impacts
execution speed.
cat test.c
void test1()
{
*(unsigned *)4 = 0x666;
*(unsigned *)(4 + 4) = 0x666;
}
void test2()
{
*(unsigned *)4 = 0x666;
*(unsigned *)(4 + 128) = 0x666;
}
void test3()
{
*(unsigned *)0x444 = 0x666;
*(unsigned *)(0x444 + 4) = 0x666;
}
void test4()
{
*(unsigned *)0x444 = 0x666;
*(unsigned *)(0x444 + 128) = 0x666;
}
void test5()
{
*(unsigned *)0x44444444 = 0x666;
*(unsigned *)(0x44444444 + 4) = 0x666;
}
void test6()
{
*(unsigned *)0x44444444 = 0x666;
*(unsigned *)(0x44444444 + 128) = 0x666;
}
arm-none-eabi-gcc -c test.c -mthumb -mcpu=cortex-m0 -mtune=cortex-m0 -Ofast -o
test.o
00000000 <test1>:
0: 2204 movs r2, #4
2: 4b02 ldr r3, [pc, #8] ; (c <test1+0xc>)
4: 6013 str r3, [r2, #0]
6: 6053 str r3, [r2, #4]
8: 4770 bx lr
a: 46c0 nop ; (mov r8, r8)
c: 00000666 .word 0x00000666
00000010 <test2>:
10: 2204 movs r2, #4
12: 4b02 ldr r3, [pc, #8] ; (1c <test2+0xc>)
14: 6013 str r3, [r2, #0]
16: 3280 adds r2, #128 ; 0x80
18: 6013 str r3, [r2, #0]
1a: 4770 bx lr
1c: 00000666 .word 0x00000666
00000020 <test3>:
20: 4b02 ldr r3, [pc, #8] ; (2c <test3+0xc>)
22: 4a03 ldr r2, [pc, #12] ; (30 <test3+0x10>)
24: 6013 str r3, [r2, #0]
26: 6053 str r3, [r2, #4]
28: 4770 bx lr
2a: 46c0 nop ; (mov r8, r8)
2c: 00000666 .word 0x00000666
30: 00000444 .word 0x00000444
00000034 <test4>:
34: 4b02 ldr r3, [pc, #8] ; (40 <test4+0xc>)
36: 4a03 ldr r2, [pc, #12] ; (44 <test4+0x10>)
38: 6013 str r3, [r2, #0]
3a: 4a03 ldr r2, [pc, #12] ; (48 <test4+0x14>)
3c: 6013 str r3, [r2, #0]
3e: 4770 bx lr
40: 00000666 .word 0x00000666
44: 00000444 .word 0x00000444
48: 000004c4 .word 0x000004c4
0000004c <test5>:
4c: 4b02 ldr r3, [pc, #8] ; (58 <test5+0xc>)
4e: 4a03 ldr r2, [pc, #12] ; (5c <test5+0x10>)
50: 6013 str r3, [r2, #0]
52: 4a03 ldr r2, [pc, #12] ; (60 <test5+0x14>)
54: 6013 str r3, [r2, #0]
56: 4770 bx lr
58: 00000666 .word 0x00000666
5c: 44444444 .word 0x44444444
60: 44444448 .word 0x44444448
00000064 <test6>:
64: 4b02 ldr r3, [pc, #8] ; (70 <test6+0xc>)
66: 4a03 ldr r2, [pc, #12] ; (74 <test6+0x10>)
68: 6013 str r3, [r2, #0]
6a: 4a03 ldr r2, [pc, #12] ; (78 <test6+0x14>)
6c: 6013 str r3, [r2, #0]
6e: 4770 bx lr
70: 00000666 .word 0x00000666
74: 44444444 .word 0x44444444
78: 444444c4 .word 0x444444c4
Compiled using:
https://launchpad.net/gcc-arm-embedded/4.9/4.9-2014-q4-major/+download/gcc-arm-none-eabi-4_9-2014q4-20141203-win32.exe
