diff options
Diffstat (limited to 'llvm/lib/Analysis')
| -rw-r--r-- | llvm/lib/Analysis/ScalarEvolution.cpp | 44 |
1 files changed, 42 insertions, 2 deletions
diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp index ef695234b66..75b427fff49 100644 --- a/llvm/lib/Analysis/ScalarEvolution.cpp +++ b/llvm/lib/Analysis/ScalarEvolution.cpp @@ -6306,8 +6306,48 @@ ScalarEvolution::HowFarToZero(const SCEV *V, const Loop *L, bool ControlsExit) { // also returns true if StepV is maximally negative (eg, INT_MIN), but that // case is not handled as this code is guarded by !CountDown. if (StepV.isPowerOf2() && - GetMinTrailingZeros(Distance) >= StepV.countTrailingZeros()) - return getUDivExactExpr(Distance, Step); + GetMinTrailingZeros(Distance) >= StepV.countTrailingZeros()) { + // Here we've constrained the equation to be of the form + // + // 2^(N + k) * Distance' = (StepV == 2^N) * X (mod 2^W) ... (0) + // + // where we're operating on a W bit wide integer domain and k is + // non-negative. The smallest unsigned solution for X is the trip count. + // + // (0) is equivalent to: + // + // 2^(N + k) * Distance' - 2^N * X = L * 2^W + // <=> 2^N(2^k * Distance' - X) = L * 2^(W - N) * 2^N + // <=> 2^k * Distance' - X = L * 2^(W - N) + // <=> 2^k * Distance' = L * 2^(W - N) + X ... (1) + // + // The smallest X satisfying (1) is unsigned remainder of dividing the LHS + // by 2^(W - N). + // + // <=> X = 2^k * Distance' URem 2^(W - N) ... (2) + // + // E.g. say we're solving + // + // 2 * Val = 2 * X (in i8) ... (3) + // + // then from (2), we get X = Val URem i8 128 (k = 0 in this case). + // + // Note: It is tempting to solve (3) by setting X = Val, but Val is not + // necessarily the smallest unsigned value of X that satisfies (3). + // E.g. if Val is i8 -127 then the smallest value of X that satisfies (3) + // is i8 1, not i8 -127 + + const auto *ModuloResult = getUDivExactExpr(Distance, Step); + + // Since SCEV does not have a URem node, we construct one using a truncate + // and a zero extend. + + unsigned NarrowWidth = StepV.getBitWidth() - StepV.countTrailingZeros(); + auto *NarrowTy = IntegerType::get(getContext(), NarrowWidth); + auto *WideTy = Distance->getType(); + + return getZeroExtendExpr(getTruncateExpr(ModuloResult, NarrowTy), WideTy); + } } // If the condition controls loop exit (the loop exits only if the expression |
