https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86847
Bug ID: 86847
Summary: [9 Regression] Switch code size growth
Product: gcc
Version: 9.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: ka6ash at gmail dot com
CC: marxin at gcc dot gnu.org
Target Milestone: ---
I noticed that the size of code has grown in switch statements between 8.1.0
and trunk. For the code shown below, gcc-trunk generates 2 jumps for each
switch statement (18 in total) when gcc-8.1.0 only 11. The difference appears
at switchlower1 pass. As I understand, gcc-8 prunes redundant tests when gcc-9
doesn't and instead relies on further tree optimizations
(https://gcc.gnu.org/ml/gcc-patches/2017-09/msg00891.html). But the
optimizations cannot manage with it as well as pruning in gcc-8 can and size
grows. Enabling -O2 does not change the situation significantly.
int cipher_to_alg(int cipher)
{
switch (cipher)
{
case 8: return 2;
case 16: return 3;
case 32: return 4;
case 64: return 6;
case 256: return 9;
case 512: return 10;
case 2048: return 11;
case 4096: return 12;
case 8192: return 13;
}
return 0;
}
arm-none-eabi-gcc-8.1.0 cipher.c -Os -S
cipher_to_alg:
cmp r0, #256
beq .L6
bgt .L3
cmp r0, #16
beq .L7
bgt .L4
cmp r0, #8
moveq r0, #2
.L15:
movne r0, #0
bx lr
.L4:
cmp r0, #32
beq .L9
cmp r0, #64
moveq r0, #6
b .L15
.L3:
cmp r0, #2048
beq .L11
bgt .L5
cmp r0, #512
moveq r0, #10
b .L15
.L5:
cmp r0, #4096
beq .L13
cmp r0, #8192
moveq r0, #13
b .L15
.L6:
mov r0, #9
bx lr
.L7:
mov r0, #3
bx lr
.L9:
mov r0, #4
bx lr
.L11:
mov r0, #11
bx lr
.L13:
mov r0, #12
bx lr
arm-none-eabi-gcc-9.0.0 cipher.c -Os -S
cipher_to_alg:
cmp r0, #256
bgt .L2
beq .L8
cmp r0, #16
bgt .L4
beq .L9
cmp r0, #8
bgt .L20
bne .L20
mov r0, #2
bx lr
.L4:
cmp r0, #32
bgt .L5
bne .L20
mov r0, #4
bx lr
.L5:
cmp r0, #64
bgt .L20
bne .L20
mov r0, #6
bx lr
.L2:
cmp r0, #2048
bgt .L6
beq .L15
cmp r0, #512
bgt .L20
bne .L20
mov r0, #10
bx lr
.L6:
cmp r0, #4096
bgt .L7
bne .L20
mov r0, #12
bx lr
.L7:
cmp r0, #8192
bgt .L20
bne .L20
mov r0, #13
bx lr
.L8:
mov r0, #9
bx lr
.L9:
mov r0, #3
bx lr
.L15:
mov r0, #11
bx lr
.L20:
mov r0, #0
bx lr
