https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102795
Bug ID: 102795
Summary: RFE: recognize !! vs branch similarity better
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: jengelh at inai dot de
Target Milestone: ---
Input
=====
unsigned long p1(unsigned long x)
{
return x - x / 10 - !!(x % 10);
}
unsigned long p2(unsigned long x)
{
if (x % 10 == 0)
return x - x / 10;
return x - x / 10 - 1;
}
unsigned long p3(unsigned long x)
{
unsigned long z = x - x / 10;
if (x % 10)
--z;
return z;
}
Outcome
=======
► ./host-x86_64-pc-linux-gnu/gcc/xg++ -B ./host-x86_64-pc-linux-gnu/gcc -c -O3
-v t.cpp
GNU C++17 (GCC) version 12.0.0 20211016 (experimental) (x86_64-pc-linux-gnu)
compiled by GNU C version 11.2.1 20210816 [revision
056e324ce46a7924b5cf10f61010cf9dd2ca10e9], GMP version 6.2.1, MPFR version
4.1.0-p7, MPC version 1.2.1, isl version none
GGC heuristics: --param ggc-min-expand=30 --param ggc-min-heapsize=4096
► objdump -Mintel -d t.o
0000000000000000 <_Z2p1m>:
...
24: 0f 95 c2 setne dl
27: 0f b6 d2 movzx edx,dl
2a: 48 29 d0 sub rax,rdx
...
0000000000000030 <_Z2p2m>:
...
4f: 48 29 d6 sub rsi,rdx
52: 48 29 d0 sub rax,rdx
55: 48 01 c9 add rcx,rcx
58: 48 39 cf cmp rdi,rcx
5b: 48 0f 44 c6 cmove rax,rsi
...
0000000000000060 <_Z2p3m>:
...
81: 48 29 d7 sub rdi,rdx
84: 48 83 ff 01 cmp rdi,0x1
88: 48 83 d0 ff adc rax,0xffffffffffffffff
...
Expected outcome
================
I would have hoped that the optimizer were able to reduce p1, p2 and p3 to the
same asm.
