https://bugs.llvm.org/show_bug.cgi?id=38492
Bug ID: 38492
Summary: Incorrect block frequencies for profile-optimized
compilation after jump threading
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: enhancement
Priority: P
Component: Scalar Optimizations
Assignee: unassignedb...@nondot.org
Reporter: dcalla...@fb.com
CC: llvm-bugs@lists.llvm.org
Created attachment 20660
--> https://bugs.llvm.org/attachment.cgi?id=20660&action=edit
test case
This bug is reduced from the spec2017 602.gcc test which is compiled with a
sample-based profiling.
After jump threading the default compilation generates an wild execution
frequency for some blocks. If jump threading is disabled
(-jump-threading-threshold=0) then more reasonable frequencies are generated.
For example, diffing the output of -debug-only=block-freq illustrates
< - while.body.i74: float = 1342177279.8, scaled = 28633115303.3, int =
28633115303
---
> - while.body.i74: float = 60.15625113, scaled = 1283.333358, int = 1283
The relative frequency of 60 is plausible while the other is not.
Jump threading updates the branch weights in two places. Curiously one place is
explicitly disabled when the there are existing profile weights while the other
explicitly requires profiling to already be present. These choices interact
poorly when the first example finds on profile branch weights and so creates
them in a context the second seems it should not have them.
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