https://llvm.org/bugs/show_bug.cgi?id=31543
Bug ID: 31543
Summary: Exponential expansion in
IndVarSimplify/ScalarEvolutionExpander
Product: tools
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: opt
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
Classification: Unclassified
Created attachment 17805
--> https://llvm.org/bugs/attachment.cgi?id=17805&action=edit
reduced ll file that gives exponential expansion with opt
File size of output file and time taken by opt is an exponential function of a
loop iteration count for the attached file:
opt -S -O1 -o foo.opt.ll foo.ll
alternatively, if -O1 is expanded and reduced:
opt -S -instcombine -loop-rotate -licm -instcombine -indvars -o foo.opt.ll
foo.ll
It's IndVarSimplify::rewriteLoopExitValues() that calls ScalarEvolutionExpander
to calculate loop exit value, but the expansion becomes exponential.
If I change the loop iteration count on this line:
%_tmp12 = icmp slt i16 %_tmp11, 6
The output ll file grows and compilation time lengthens exponentially. Some
sample values:
bound file size of foo.opt.ll
6 4912
7 8944
8 17088
..
15 2065266
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