[llvm] [LAA] Use computeConstantDifference() (PR #103725)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-analysis

Author: Nikita Popov (nikic)

<details>
<summary>Changes</summary>

Use computeConstantDifference() instead of casting getMinusSCEV() to SCEVConstant. This can be much faster in some cases, because computeConstantDifference() computes the result without creating new SCEV expressions.

This improves LTO/ThinLTO compile-time for lencod by more than 10%, see: http://llvm-compile-time-tracker.com/compare.php?from=6da3361f504495cef71caa4de4297234b6ea7fc7&to=b24e3c3338f62214d9abec8c9ef5a502953e7748&stat=instructions%3Au

I've verified that computeConstantDifference() does not produce worse results than the previous code for anything in llvm-test-suite. This required raising the iteration cutoff to 6. I ended up increasing it to 8 just to be on the safe side (for code outside llvm-test-suite), and because this doesn't materially affect compile-time anyway (we'll almost always bail out earlier).

---
Full diff: https://github.com/llvm/llvm-project/pull/103725.diff


2 Files Affected:

- (modified) llvm/lib/Analysis/LoopAccessAnalysis.cpp (+3-3) 
- (modified) llvm/lib/Analysis/ScalarEvolution.cpp (+1-1) 


``````````diff
diff --git a/llvm/lib/Analysis/LoopAccessAnalysis.cpp b/llvm/lib/Analysis/LoopAccessAnalysis.cpp
index d67fd7985f6a5..8c939f261e693 100644
--- a/llvm/lib/Analysis/LoopAccessAnalysis.cpp
+++ b/llvm/lib/Analysis/LoopAccessAnalysis.cpp
@@ -1599,11 +1599,11 @@ std::optional<int> llvm::getPointersDiff(Type *ElemTyA, Value *PtrA,
     // Otherwise compute the distance with SCEV between the base pointers.
     const SCEV *PtrSCEVA = SE.getSCEV(PtrA);
     const SCEV *PtrSCEVB = SE.getSCEV(PtrB);
-    const auto *Diff =
-        dyn_cast<SCEVConstant>(SE.getMinusSCEV(PtrSCEVB, PtrSCEVA));
+    std::optional<APInt> Diff =
+        SE.computeConstantDifference(PtrSCEVB, PtrSCEVA);
     if (!Diff)
       return std::nullopt;
-    Val = Diff->getAPInt().getSExtValue();
+    Val = Diff->getSExtValue();
   }
   int Size = DL.getTypeStoreSize(ElemTyA);
   int Dist = Val / Size;
diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index af341c55205de..71a72fad35b32 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -11956,7 +11956,7 @@ ScalarEvolution::computeConstantDifference(const SCEV *More, const SCEV *Less) {
   APInt DiffMul(BW, 1);
   // Try various simplifications to reduce the difference to a constant. Limit
   // the number of allowed simplifications to keep compile-time low.
-  for (unsigned I = 0; I < 5; ++I) {
+  for (unsigned I = 0; I < 8; ++I) {
     if (More == Less)
       return Diff;
 

``````````

</details>


https://github.com/llvm/llvm-project/pull/103725