[PATCH] D19002: [LazyValueInfo] Fix for a nasty compile-time problem with questions

[Gerolf Hoflehner via llvm-commits]

Gerolf updated this revision to Diff 54314.
Gerolf added a comment.

Mostly removed some of my questions and added a few comments based on Philip's
review.


http://reviews.llvm.org/D19002

Files:
  lib/Analysis/LazyValueInfo.cpp

Index: lib/Analysis/LazyValueInfo.cpp
===================================================================
--- lib/Analysis/LazyValueInfo.cpp
+++ lib/Analysis/LazyValueInfo.cpp
@@ -60,6 +60,10 @@
 /// FIXME: This is basically just for bringup, this can be made a lot more rich
 /// in the future.
 ///
+/// Note:
+/// constantrange applies only to integers. constant/notconstant applies to
+/// non-integers, too.
+
 namespace {
 class LVILatticeVal {
   enum LatticeValueTy {
@@ -240,6 +244,7 @@
 
     if (isNotConstant()) {
       if (RHS.isConstant()) {
+        // A value is different on two paths -> mark as bottom (overdefined)
         if (Val == RHS.Val)
           return markOverdefined();
 
@@ -748,6 +753,29 @@
     }
   }
 
+  // FIXME: this is to solve a compile-time problem.
+  // In a test case with thousands of alloca's and
+  // indiect calls the solver pushed the allocas as undefined
+  // on the stack and tries to "solve" them. This seems to
+  // triggered that the value range problem in the current algorithm
+  // is not solved as a forward problem with well defined top, bottom, meet
+  // etc. Instead it may walk the CFG spontaneously in any direction and push
+  // values on the block stack while it tries to determine the lattice values
+  // of the values on the stack. This is illustrated in the current routine:
+  // solve -> solveBlockValue -> solveBlockValueNonLocal, which (see below)
+  // may call getEdgeValue, which in turn could push more values on the stack
+  // This code fixes the test cases with the alloca's reducing compile-time
+  // for that ~150K line funciton from "inifinite" to a couple of minutes.
+  // The rational is that "not null" is the best we can do for a stackaddress.
+
+  if (isa<AllocaInst>(Val)) {
+    assert(NotNull && "Stackaddress cannot be zero\n");
+    PointerType *PTy = cast<PointerType>(Val->getType());
+    Result = LVILatticeVal::getNot(ConstantPointerNull::get(PTy));
+    BBLV = Result;
+    return true;
+  }
+
   // If this is the entry block, we must be asking about an argument.  The
   // value is overdefined.
   if (BB == &BB->getParent()->getEntryBlock()) {


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