[PATCH] D28582: [SCEV] Simplify SolveLinEquationWithOverflow a bit.

[Eli Friedman via Phabricator via llvm-commits]

efriedma created this revision.
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Cleanup in preparation for generalizing it.


Repository:
  rL LLVM

https://reviews.llvm.org/D28582

Files:
  lib/Analysis/ScalarEvolution.cpp


Index: lib/Analysis/ScalarEvolution.cpp
===================================================================
--- lib/Analysis/ScalarEvolution.cpp
+++ lib/Analysis/ScalarEvolution.cpp
@@ -7037,15 +7037,16 @@
   APInt AD = A.lshr(Mult2).zext(BW + 1);  // AD = A / D
   APInt Mod(BW + 1, 0);
   Mod.setBit(BW - Mult2);  // Mod = N / D
-  APInt I = AD.multiplicativeInverse(Mod);
+  // I is guaranteed to fit into BW bits, so truncate it.
+  APInt I = AD.multiplicativeInverse(Mod).trunc(BW);
 
   // 4. Compute the minimum unsigned root of the equation:
   // I * (B / D) mod (N / D)
-  APInt Result = (I * B.lshr(Mult2).zext(BW + 1)).urem(Mod);
+  // To simplify the computation, we factor out the divide by D:
+  // (I * B mod N) / D
+  APInt Result = (I * B).lshr(Mult2);
 
-  // The result is guaranteed to be less than 2^BW so we may truncate it to BW
-  // bits.
-  return SE.getConstant(Result.trunc(BW));
+  return SE.getConstant(Result);
 }
 
 /// Find the roots of the quadratic equation for the given quadratic chrec


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