https://bugs.llvm.org/show_bug.cgi?id=42771
Bug ID: 42771
Summary: quadratic compile time in -instsimplify
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: Scalar Optimizations
Assignee: unassignedb...@nondot.org
Reporter: jay.f...@gmail.com
CC: llvm-bugs@lists.llvm.org
Created attachment 22292
--> https://bugs.llvm.org/attachment.cgi?id=22292&action=edit
test case
With the attached test case I get:
$ time ~/llvm-release/bin/opt -instsimplify stripped.ll -o /dev/null
real 0m0.849s
user 0m0.840s
sys 0m0.008s
This is a long time for running -instsimplify on about 8000 instructions! The
reason is that r207302 (https://reviews.llvm.org/rL207302) introduced this code
in instsimplifys' runImpl():
// RecursivelyDeleteTriviallyDeadInstruction can remove
// more than one instruction, so simply incrementing the
// iterator does not work. When instructions get deleted
// re-iterate instead.
BI = BB->begin(); BE = BB->end();
which causes the iteration over instructions in a basic block to take quadratic
time. On my test case, simply removing this line gives a ~30x speed-up:
$ time ~/llvm-release/bin/opt -instsimplify stripped.ll -o /dev/null
real 0m0.030s
user 0m0.025s
sys 0m0.004s
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