[PATCH] D28308: [SCEV] Model ashr(shl(x, n), m) as mul(x, 2^(n-m)) when n > m

Eli Friedman via Phabricator via llvm-commits llvm-commits at lists.llvm.org
Mon Jan 16 16:51:04 PST 2017

efriedma added inline comments.

Comment at: lib/Analysis/ScalarEvolution.cpp:5271
+          if (CI->isNullValue())
+            return LShOp0SCEV; // shift by zero --> noop
I think you meant to return `getSCEV(L)` here?  Please add a testcase for `(ashr (shl x, 1) 0)`.

Comment at: lib/Analysis/ScalarEvolution.cpp:5304
+          const SCEV *MulSCEV = getSCEV(
+              ConstantInt::get(TruncTy, Mul.getZExtValue()));
+          return getSignExtendExpr(
64 might not be large enough, and getZExtValue() will assert if the value doesn't fit into a 64-bit integer.  The right code is something like `APInt Mul = APInt::getOneBitSet(TruncToWidth, LShAmt - AShrAmt); const SCEV *MulSCEV = getConstant(Mul);`.  Please add a testcase like `(ashr (shl i128 x, 100) 1)`  to show this works correctly.



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