[llvm] r328543 - [Pipeliner] Fix number of phis to generate in the epilog

[Krzysztof Parzyszek via llvm-commits]

Author: kparzysz
Date: Mon Mar 26 09:37:55 2018
New Revision: 328543

URL: http://llvm.org/viewvc/llvm-project?rev=328543&view=rev
Log:
[Pipeliner] Fix number of phis to generate in the epilog

The pipeliner was generating too many phis in the epilog blocks, which
caused incorrect code generation when rewriting an instruction that uses
the phi.

In this case, there 3 prolog and epilog stages. An existing phi was
scheduled at stage 1. When generating the code for the 2nd epilog an
extra new phi was generated.

To fix this, we need to update the code that calculates the maximum
number of phis that can be generated, which is based upon the current
prolog stage and the stage of the original phi. In this case, when the
prolog stage is 1 and the original phi stage is 1, the maximum number
of phis to generate is 2.

Patch by Brendon Cahoon.

Modified:
    llvm/trunk/lib/CodeGen/MachinePipeliner.cpp

Modified: llvm/trunk/lib/CodeGen/MachinePipeliner.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/MachinePipeliner.cpp?rev=328543&r1=328542&r2=328543&view=diff
==============================================================================
--- llvm/trunk/lib/CodeGen/MachinePipeliner.cpp (original)
+++ llvm/trunk/lib/CodeGen/MachinePipeliner.cpp Mon Mar 26 09:37:55 2018
@@ -2629,16 +2629,13 @@ void SwingSchedulerDAG::generateExisting
         VRMap[CurStageNum][Def] = VRMap[CurStageNum][LoopVal];
     }
     // Adjust the number of Phis needed depending on the number of prologs left,
-    // and the distance from where the Phi is first scheduled.
-    unsigned NumPhis = NumStages;
-    if (!InKernel && (int)PrologStage < LoopValStage)
-      // The NumPhis is the maximum number of new Phis needed during the steady
-      // state. If the Phi has not been scheduled in current prolog, then we
-      // need to generate less Phis.
-      NumPhis = std::max((int)NumPhis - (int)(LoopValStage - PrologStage), 1);
-    // The number of Phis cannot exceed the number of prolog stages. Each
-    // stage can potentially define two values.
-    NumPhis = std::min(NumPhis, PrologStage + 2);
+    // and the distance from where the Phi is first scheduled. The number of
+    // Phis cannot exceed the number of prolog stages. Each stage can
+    // potentially define two values.
+    unsigned MaxPhis = PrologStage + 2;
+    if (!InKernel && (int)PrologStage <= LoopValStage)
+      MaxPhis = std::max((int)MaxPhis - (int)LoopValStage, 1);
+    unsigned NumPhis = std::min(NumStages, MaxPhis);
 
     unsigned NewReg = 0;