https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81346
Bug ID: 81346
Summary: Missed constant propagation into comparison
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: gergo.barany at inria dot fr
Target Milestone: ---
Created attachment 41694
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=41694&action=edit
Input C file for triggering the issue
The attached C file contains the following function:
int fn1(int p1) {
int b = (p1 / 12 == 6);
return b;
}
As these are integers, the expression (p1 / 12 == 6) can be optimized to a
subtraction and an unsigned compare. GCC can do this (here for ARM):
fn1:
sub r0, r0, #72
cmp r0, #11
movhi r0, #0
movls r0, #1
bx lr
The attached file also contains the following function:
int fn2(int p1) {
int a = 6;
int b = (p1 / 12 == a);
return b;
}
This is equivalent to the above code; the value of a can only ever be 6.
Consequently, the output machine code should be equivalent. However, GCC does
not recognize the above pattern and generates more complex code:
fn2:
movw r3, #43691
movt r3, 10922
smull r2, r3, r3, r0
asr r0, r0, #31
rsb r0, r0, r3, asr #1
sub r0, r0, #6
clz r0, r0
lsr r0, r0, #5
bx lr
I believe this is a target-independent optimization issue because x86-64 and
PowerPC behave analogously, for example (x86-64):
fn1:
subl $72, %edi
xorl %eax, %eax
cmpl $11, %edi
setbe %al
ret
fn2:
movl %edi, %eax
movl $715827883, %edx
sarl $31, %edi
imull %edx
xorl %eax, %eax
sarl %edx
subl %edi, %edx
cmpl $6, %edx
sete %al
ret
Version:
gcc version 8.0.0 20170706 (experimental) (GCC)
Configured with: --target=armv7a-eabihf --with-arch=armv7-a
--with-fpu=vfpv3-d16 --with-float-abi=hard --with-float=hard
or
with: --target=x86_64-pc-linux-gnu
or
with: --target=ppc-eabi
