https://bugs.llvm.org/show_bug.cgi?id=45690
Bug ID: 45690
Summary: Slow code (-O2) due to using jne instead of using
cmove
Product: clang
Version: 10.0
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: C++
Assignee: unassignedclangb...@nondot.org
Reporter: levo.delel...@gmail.com
CC: blitzrak...@gmail.com, dgre...@apple.com,
erik.pilking...@gmail.com, llvm-bugs@lists.llvm.org,
richard-l...@metafoo.co.uk
Created attachment 23403
--> https://bugs.llvm.org/attachment.cgi?id=23403&action=edit
The test cpp file
I attached my test file. I compiled with clang -O2 test.cpp -S
I notice branching in my code when I specifically wrote it so it wouldn't
branch. It jumped over a single instruction so I replaced it with a cmove. It
appears to cut this loop time in half
95,97c95
< jne .LBB1_8
< # %bb.7: # in Loop: Header=BB1_6 Depth=1
< movl %ecx, %edx
---
> cmove %ecx, %edx
105,107c103
< jne .LBB1_10
< # %bb.9: # in Loop: Header=BB1_6 Depth=1
< movl %ecx, %edx
---
> cmove %ecx, %edx
115,117c111
< jne .LBB1_12
< # %bb.11: # in Loop: Header=BB1_6 Depth=1
< movl %ebx, %edx
---
> cmove %ebx, %edx
125,127c119
< jne .LBB1_14
< # %bb.13: # in Loop: Header=BB1_6 Depth=1
< movl %ecx, %edx
---
> cmove %ecx, %edx
135,137c127
< jne .LBB1_16
< # %bb.15: # in Loop: Header=BB1_6 Depth=1
< movl %ebx, %edx
---
> cmove %ebx, %edx
145,147c135
< jne .LBB1_18
< # %bb.17: # in Loop: Header=BB1_6 Depth=1
< movl %ecx, %edx
---
> cmove %ecx, %edx
155,157c143
< jne .LBB1_20
< # %bb.19: # in Loop: Header=BB1_6 Depth=1
< movl %ebx, %edx
---
> cmove %ebx, %edx
165,167c151
< jne .LBB1_22
< # %bb.21: # in Loop: Header=BB1_6 Depth=1
< movl %ecx, %edx
---
> cmove %ecx, %edx
--
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