[Unrolling] Refactor the start and step offsets to simplify overflow

checking and make the cache faster and smaller.

I had thought that using an APInt here would be useful, but I think
I was just wrong. Notably, we don't have to do any fancy overflow
checking, we can just bound the values as quite small and do the math in
a higher precision integer. I've switched to a signed integer so that
UBSan will even point out if we ever have integer overflow. I've added
various asserts to try to catch things as well and hoisted the overflow
checks so that we just leave the too-large offsets out of the SCEV-GEP
cache. This makes the value in the cache quite a bit smaller which is
probably worthwhile.

No functionality changed here (for trip counts under 1 billion).

llvm-svn: 237209

1 files changed, 26 insertions, 10 deletions

diff --git a/llvm/lib/Transforms/Scalar/LoopUnrollPass.cpp b/llvm/lib/Transforms/Scalar/LoopUnrollPass.cpp
index f65b9541d58..03e1f4cbf60 100644
--- a/llvm/lib/Transforms/Scalar/LoopUnrollPass.cpp
+++ b/llvm/lib/Transforms/Scalar/LoopUnrollPass.cpp
@@ -340,8 +340,8 @@ class UnrollAnalyzer : public InstVisitor<UnrollAnalyzer, bool> {
 
   struct SCEVGEPDescriptor {
     Value *BaseAddr;
-    APInt Start;
-    APInt Step;
+    unsigned Start;
+    unsigned Step;
   };
 
   /// \brief The loop we're going to analyze.
@@ -435,12 +435,12 @@ class UnrollAnalyzer : public InstVisitor<UnrollAnalyzer, bool> {
     if (!CDS)
       return false;
 
-    // Check possible overflow.
-    if (GEPDesc.Start.getActiveBits() > 32 || GEPDesc.Step.getActiveBits() > 32)
-      return false;
-    unsigned ElemSize = CDS->getElementType()->getPrimitiveSizeInBits() / 8U;
-    uint64_t Index = (GEPDesc.Start.getLimitedValue() +
-                      GEPDesc.Step.getLimitedValue() * Iteration) /
+    // This calculation should never overflow because we bound Iteration quite
+    // low and both the start and step are 32-bit integers. We use signed
+    // integers so that UBSan will catch if a bug sneaks into the code.
+    int ElemSize = CDS->getElementType()->getPrimitiveSizeInBits() / 8U;
+    int64_t Index = ((int64_t)GEPDesc.Start +
+                      (int64_t)GEPDesc.Step * (int64_t)Iteration) /
                      ElemSize;
     if (Index >= CDS->getNumElements()) {
       // FIXME: For now we conservatively ignore out of bound accesses, but
@@ -495,8 +495,17 @@ class UnrollAnalyzer : public InstVisitor<UnrollAnalyzer, bool> {
           if (!StepSE || !StartSE)
             continue;
 
-          SCEVCache[V] = {Visitor.BaseAddress, StartSE->getValue()->getValue(),
-                          StepSE->getValue()->getValue()};
+          // Check and skip caching if doing so would require lots of bits to
+          // avoid overflow.
+          APInt Start = StartSE->getValue()->getValue();
+          APInt Step = StepSE->getValue()->getValue();
+          if (Start.getActiveBits() > 32 || Step.getActiveBits() > 32)
+            continue;
+
+          // We found a cacheable SCEV model for the GEP.
+          SCEVCache[V] = {Visitor.BaseAddress,
+                          (unsigned)Start.getLimitedValue(),
+                          (unsigned)Step.getLimitedValue()};
         }
       }
     }
@@ -529,6 +538,13 @@ public:
   bool analyzeLoop() {
     SmallSetVector<BasicBlock *, 16> BBWorklist;
 
+    // We want to be able to scale offsets by the trip count and add more
+    // offsets to them without checking for overflows, and we already don't want
+    // to analyze *massive* trip counts, so we force the max to be reasonably
+    // small.
+    assert(UnrollMaxIterationsCountToAnalyze < (INT_MAX / 2) &&
+           "The unroll iterations max is too large!");
+
     // Don't simulate loops with a big or unknown tripcount
     if (!UnrollMaxIterationsCountToAnalyze || !TripCount ||
         TripCount > UnrollMaxIterationsCountToAnalyze)