https://llvm.org/bugs/show_bug.cgi?id=27632
Bug ID: 27632
Summary: SLP vectorizer is highly dependent on the operand
order
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Keywords: googler, performance
Severity: normal
Priority: P
Component: Loop Optimizer
Assignee: unassignedb...@nondot.org
Reporter: daniel...@gmail.com
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
Created attachment 16302
--> https://llvm.org/bugs/attachment.cgi?id=16302&action=edit
loads that cannot be vectorized
In the attached IR, the only difference between 1.ll and 2.ll is the order of
the add opeands, e.g.
33c33
< %24 = add nsw i32 %23, %15
---
> %24 = add nsw i32 %15, %23
However, this will lead to SLPVectorizer being unable to vectorize the basic
block.
#opt -slp-vectorizer 1.ll -S |grep load
%9 = load i8, i8* %.08.i, align 1
%10 = load i8, i8* %.036.i, align 1
%12 = load i8, i8* %11, align 1
%14 = load i8, i8* %13, align 1
%16 = load i8, i8* %15, align 1
%18 = load i8, i8* %17, align 1
%20 = load i8, i8* %19, align 1
%22 = load i8, i8* %21, align 1
%24 = load i8, i8* %23, align 1
%26 = load i8, i8* %25, align 1
%28 = load i8, i8* %27, align 1
%30 = load i8, i8* %29, align 1
%32 = load i8, i8* %31, align 1
%34 = load i8, i8* %33, align 1
%36 = load i8, i8* %35, align 1
%47 = load i8, i8* %46, align 1
#opt -slp-vectorizer x.ll -S |grep load
%23 = load <8 x i8>, <8 x i8>* %22, align 1
%27 = load <8 x i8>, <8 x i8>* %26, align 1
This is because when building the vectorization tree, it uses post-order to
traverse the reduction operations, and visits its children in the order of its
index. So if the index of the operand change, load will need to be processed in
reverse-order, thus cannot be vectorized.
Note that the operand could be any order after reassociation pass. Thus it
important that SLPVectorizer can handle any operand order.
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