[LVI] Improve select handling to use condition

This patches teaches LVI to recognize clamp idioms (e.g. select(a > 5, a, 5) will always produce something greater than 5.

The tests end up being somewhat simplistic because trying to exercise the case I actually care about (a loop with a range check on a clamped secondary induction variable) ends up tripping across a couple of other imprecisions in the analysis. Ah, the joys of LVI...

Differential Revision: http://reviews.llvm.org/D16827

llvm-svn: 260627

1 files changed, 19 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/LazyValueInfo.cpp b/llvm/lib/Analysis/LazyValueInfo.cpp
index bc90b929cf0..8f121110051 100644
--- a/llvm/lib/Analysis/LazyValueInfo.cpp
+++ b/llvm/lib/Analysis/LazyValueInfo.cpp
@@ -911,6 +911,25 @@ bool LazyValueInfoCache::solveBlockValueSelect(LVILatticeVal &BBLV,
     return true;
   }
 
+  // Can we constrain the facts about the true and false values by using the
+  // condition itself?  This shows up with idioms like e.g. select(a > 5, a, 5).
+  // TODO: We could potentially refine an overdefined true value above.
+  if (auto *ICI = dyn_cast<ICmpInst>(SI->getCondition())) {
+    LVILatticeVal TrueValTaken, FalseValTaken;
+    if (!getValueFromFromCondition(SI->getTrueValue(), ICI,
+                                   TrueValTaken, true))
+      TrueValTaken.markOverdefined();
+    if (!getValueFromFromCondition(SI->getFalseValue(), ICI,
+                                   FalseValTaken, false))
+      FalseValTaken.markOverdefined();
+
+    TrueVal = intersect(TrueVal, TrueValTaken);
+    FalseVal = intersect(FalseVal, FalseValTaken);
+  }
+
+  // TODO: handle idioms like min & max where we can use a more precise merge
+  // when our inputs are constant ranges.
+  
   LVILatticeVal Result;  // Start Undefined.
   Result.mergeIn(TrueVal, DL);
   Result.mergeIn(FalseVal, DL);