From: Arthur Perais <arthur.perais@inria.fr>
Date: Wed, 21 Dec 2016 21:05:24 +0000 (-0600)
Subject: cpu: Resolve targets of predicted 'taken' decode for O3
X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=1664625db89c3c9054434b5dc97a9f1c1bfad244;p=gem5.git

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 <jason@lowepower.com>
---

diff --git a/src/cpu/o3/decode_impl.hh b/src/cpu/o3/decode_impl.hh
index ca56ac1e4..15ec76f5a 100644
--- a/src/cpu/o3/decode_impl.hh
+++ b/src/cpu/o3/decode_impl.hh
@@ -719,7 +719,11 @@ DefaultDecode<Impl>::decodeInsts(ThreadID tid)
         }
 
         // 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())) {