Add debug option to turn off zero cost phi node inserting in PRE

[• hulx2000]

Summary

commit 403050abcc091260be2e8f58484e7a39c0782b47
Author: Daniel Berlin <[e-mail]>
Date: Tue Feb 3 20:37:08 2015 +0000

Allow PRE to insert no-cost phi nodes

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@228024 91177308-0d34-0410-b5e6-96231b3b80d8

Have very broad performance impact to our benchmark, the degradation are up to 30%.

According to owner of this patch, there is no easy fix for now, so I created this patch to add a debug option to allow us to turn this feature off with this debug option, the debug option is off by default.

[• hulx2000]

Opened a bug Bug 24171 - Load PRE inserts badly placed fully-redundant address arithmetic for it Daniel is working on the right solution.

lib/Transforms/Scalar/GVN.cpp

 STATISTIC(NumGVNBlocks, "Number of blocks merged");
 STATISTIC(NumGVNSimpl,  "Number of instructions simplified");
 STATISTIC(NumGVNEqProp, "Number of equalities propagated");
 STATISTIC(NumPRELoad,   "Number of loads PRE'd");
 
 static cl::opt<bool> EnablePRE("enable-pre",
                                cl::init(true), cl::Hidden);
 static cl::opt<bool> EnableLoadPRE("enable-load-pre", cl::init(true));
+static cl::opt<bool> DisableZeroCostPHIPRE("disable-phi-pre", cl::init(false));
 
 // Maximum allowed recursion depth.
 static cl::opt<uint32_t>
 MaxRecurseDepth("max-recurse-depth", cl::Hidden, cl::init(1000), cl::ZeroOrMore,
                 cl::desc("Max recurse depth (default = 1000)"));
 
 //===----------------------------------------------------------------------===//
 //                         ValueTable Class
for (pred_iterator PI = pred_begin(CurrentBlock), PE = pred_end(CurrentBlock);
     } else {
       predMap.push_back(std::make_pair(predV, P));
       ++NumWith;
     }
   }
 
   // Don't do PRE when it might increase code size, i.e. when
   // we would need to insert instructions in more than one pred.
-  if (NumWithout > 1 || NumWith == 0)
+  if ((DisableZeroCostPHIPRE && NumWithout != 1) || 
+      ((!DisableZeroCostPHIPRE) && NumWithout > 1) || NumWith == 0)
     return false;
 
   // We may have a case where all predecessors have the instruction,
   // and we just need to insert a phi node. Otherwise, perform
   // insertion.
   Instruction *PREInstr = nullptr;
 
   if (NumWithout != 0) {