https://bugs.llvm.org/show_bug.cgi?id=32022
Bug ID: 32022
Summary: Multiple simplifycfg pass make some loops
significantly slower
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: normal
Priority: P
Component: Loop Optimizer
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
Created attachment 18011
--> https://bugs.llvm.org/attachment.cgi?id=18011&action=edit
sample C code to demonstrate problem
With `opt -simplifycfg -sroa -simplifcfy`, loops with a if-branch in the body
gets >2x slower if they have a single backedge. I have attached C source code
that can demonstrate this problem with clang version 3.9, 4.0 and
5.0.0-svn295492-1~exp1 from the nightly build. (I haven't tested earlier
versions.) In the code, the apple() function is the slow version and the
orange() function is the fast version of the same code. Their only difference
is the location for the `++i` loop increment.
I speculate that the problem comes from the select instructions created by the
2nd simplifycfg:
%all_missing.1 = select i1 %7, i32 0, i32 %all_missing.0
%amin.1 = select i1 %7, double %6, double %amin.0
which probably corresponds to the following x86 assembly:
movaps %xmm1,%xmm2
cmpnlesd %xmm0,%xmm2
andps %xmm2,%xmm0
andnps %xmm1,%xmm2
orps %xmm2,%xmm0
We previously thought this problem is related to bug #30452 but we were
recommended to open a new issue.
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