http://llvm.org/bugs/show_bug.cgi?id=11388
Bug #: 11388
Summary: Allow getSCEVAtScope/ComputeBackEdgeTakenCount to
handle multiple loop exits
Product: libraries
Version: trunk
Platform: PC
OS/Version: All
Status: ASSIGNED
Severity: enhancement
Priority: P
Component: Loop Optimizer
AssignedTo: [email protected]
ReportedBy: [email protected]
CC: [email protected]
Depends on: 11375
Classification: Unclassified
The fix for PR11375 potentially disabled some optimization. Enabling this
optimization requires updating uses of getSCEVAtScope to indicate which loop
exit it wants the value for. I did not see significant benchmark regressions on
x86 or ARM, but two unit tests have been XFAIL'd until this optimization is
reimplemented:
Transforms/IndVarSimplify/loop_evaluate9.ll and loop_evaluate10.ll.
Additionally, ComputeBackEdgeTakenCount can be smarter. Some cases we could be
be optimistic with are: (1) the loop has unit stride (2) the last loop test is
less-than/greater-than (3) any loop test is less-than/greater-than AND
falls-through some constant times less then the other tests.
This applies to both BECount and MaxBECount.
Note that "back-edge-taken count" is the number of times the loop latch
branches to the loop header. This depends on all loop exit tests. Whereas the
"exit-not-taken count" is specific to a particular loop exit and only
applicable if the loop actually takes that exit.
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