From: Santi Galan <santi.galan@metempsy.com>
Date: Tue, 18 Apr 2017 16:08:46 +0000 (-0500)
Subject: x86: fixed branching() computation for branch uops
X-Git-Tag: v19.0.0.0~2844
X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=3cc6df46a3cd0aa97cd960f7f284d21c36a05181;p=gem5.git

x86: fixed branching() computation for branch uops

When a branch micro-op belongs to a flow and the micro-op does not change
the nPC and just updates the nuPC (like a 'rep movs' flow), branching()
function always returns not-taken no matter  actual micro-branch outcome.
Provided fix adds to the equation  nuPC attribute checking since these kind
of branch micro-op only updates that pointer.

This issue has been found while debugging the performance of a copy-loop
implemented with memcopy function. Without the fix, 'rep movss' internal
micro-branch was always predicted as not-taken causing an squash event
after every branch micro-branch execution.

Using the provided test, branch mispredition went from 1922 without the fix
to 7.

Change-Id: I1bcbefae26aef47e3135817ef99b53d0ea0a98fa
---

diff --git a/src/arch/x86/types.hh b/src/arch/x86/types.hh
index 954f9f16e..8d47b7efb 100644
--- a/src/arch/x86/types.hh
+++ b/src/arch/x86/types.hh
@@ -318,7 +318,8 @@ namespace X86ISA
         bool
         branching() const
         {
-            return this->npc() != this->pc() + size();
+            return (this->npc() != this->pc() + size()) ||
+                   (this->nupc() != this->upc() + 1);
         }
 
         void