changeset 1ae84c76066b in /z/repo/gem5
details: http://repo.gem5.org/gem5?cmd=changeset;node=1ae84c76066b
description:
cpu: Resolve targets of predicted 'taken' decode for O3
The target of taken conditional direct branches does not
need to be resolved in IEW: the target can be computed at
decode, usually using the decoded instruction word and the PC.
The higher-than-necessary penalty is taken only on conditional
branches that are predicted taken but miss in the BTB. Thus,
this is mostly inconsequential on IPC if the BTB is big/associative
enough (fewer capacity/conflict misses). Nonetheless, what gem5
simulates is not representative of how conditional branch targets
can be handled.
Signed-off-by: Jason Lowe-Power <ja...@lowepower.com>
diffstat:
src/cpu/o3/decode_impl.hh | 6 +++++-
1 files changed, 5 insertions(+), 1 deletions(-)
diffs (16 lines):
diff -r 9af039ea0c1e -r 1ae84c76066b src/cpu/o3/decode_impl.hh
--- a/src/cpu/o3/decode_impl.hh Wed Dec 21 15:04:06 2016 -0600
+++ b/src/cpu/o3/decode_impl.hh Wed Dec 21 15:05:24 2016 -0600
@@ -719,7 +719,11 @@
}
// Go ahead and compute any PC-relative branches.
- if (inst->isDirectCtrl() && inst->isUncondCtrl()) {
+ // This includes direct unconditional control and
+ // direct conditional control that is predicted taken.
+ if (inst->isDirectCtrl() &&
+ (inst->isUncondCtrl() || inst->readPredTaken()))
+ {
++decodeBranchResolved;
if (!(inst->branchTarget() == inst->readPredTarg())) {
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