https://bugs.llvm.org/show_bug.cgi?id=39235
Bug ID: 39235
Summary: [loop vectorize] failure to vectorize after GVN
load-pre
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: enhancement
Priority: P
Component: Loop Optimizer
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
Created attachment 20981
--> https://bugs.llvm.org/attachment.cgi?id=20981&action=edit
test showing the problem
I'm seeing an issue where adding !noalias metadata seems to prevents loop
vectorization from occurring. The issue seems to be that GVN does a PRE of a
load and this prevents the loop vectorizer from functioning.
See the attached .ll file. Supposing it's named opt-me.ll, to reproduce you can
do this:
opt -O3 -loop-vectorize -force-vector-width=4 -force-vector-interleave=1
opt-me.ll -S
Result:
remark: <unknown>:0:0: loop not vectorized: value that could not be identified
as reduction is used outside the loop
remark: <unknown>:0:0: loop not vectorized: value that could not be identified
as reduction is used outside the loop
-> loop not vectorized
Compare with
opt -O3 -loop-vectorize -force-vector-width=4 -force-vector-interleave=1
-enable-load-pre=false opt-me.ll -S
-> loop is vectorized, I see 4 x i32, extractelements, and no loop not
vectorized remarks.
If you remove the !noalias metadata from opt-me.ll, vectorization occurs
whether or not -enable-load-pre=false is passed. This seems unfortunate, as
front-ends would like to provide !noalias metadata where possible to improve
(and not hinder!) optimization.
I believe this problem is present in LLVM 6 as well as master.
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