https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77484
Bug ID: 77484
Summary: Static branch predictor causes ~6-8% regression of
SPEC2000 GAP
Product: gcc
Version: 6.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: wdijkstr at arm dot com
Target Milestone: ---
Changes in the static branch predictor (around August last year) caused
regressions on SPEC2000. The PRED_CALL predictor causes GAP to regress by 6-8%
on AArch64, and this hasn't been fixed on trunk. With this predictor turned
off, INT is 0.6% faster and FP 0.4%.
The reason is that the predictor causes calls that are guarded by if-statements
to be placed at the end of the function. For Gap this is bad as it often
executes several such statements in a row, resulting in 2 extra taken branches
and additional I-cache misses per if-statement. So it seems that on average
this prediction makes things worse.
Overall the static prediction and -freorder-blocks provide a benefit. However
does the gain of each static prediction being correct outweigh the cost of the
prediction being incorrect? Has this been measured for each of the static
predictors across multiple targets?
