[llvm] [LoopVectorize] Vectorize select-cmp reduction pattern for increasing integer induction variable (PR #67812)
Mel Chen via llvm-commits
llvm-commits at lists.llvm.org
Thu Oct 31 06:02:49 PDT 2024
================
@@ -659,6 +661,100 @@ RecurrenceDescriptor::isAnyOfPattern(Loop *Loop, PHINode *OrigPhi,
: RecurKind::FAnyOf);
}
+// We are looking for loops that do something like this:
+// int r = 0;
+// for (int i = 0; i < n; i++) {
+// if (src[i] > 3)
+// r = i;
+// }
+// The reduction value (r) is derived from either the values of an increasing
+// induction variable (i) sequence, or from the start value (0).
+// The LLVM IR generated for such loops would be as follows:
+// for.body:
+// %r = phi i32 [ %spec.select, %for.body ], [ 0, %entry ]
+// %i = phi i32 [ %inc, %for.body ], [ 0, %entry ]
+// ...
+// %cmp = icmp sgt i32 %5, 3
+// %spec.select = select i1 %cmp, i32 %i, i32 %r
+// %inc = add nsw i32 %i, 1
+// ...
+// Since 'i' is an increasing induction variable, the reduction value after the
+// loop will be the maximum value of 'i' that the condition (src[i] > 3) is
+// satisfied, or the start value (0 in the example above). When the start value
+// of the increasing induction variable 'i' is greater than the minimum value of
+// the data type, we can use the minimum value of the data type as a sentinel
+// value to replace the start value. This allows us to perform a single
+// reduction max operation to obtain the final reduction result.
+// TODO: It is possible to solve the case where the start value is the minimum
+// value of the data type or a non-constant value by using mask and multiple
+// reduction operations.
+RecurrenceDescriptor::InstDesc
+RecurrenceDescriptor::isFindLastIVPattern(PHINode *OrigPhi, Instruction *I,
+ ScalarEvolution &SE) {
+ // TODO: Support the vectorization of FindLastIV when the reduction phi is
+ // used by more than one select instruction. This vectorization is only
+ // performed when the SCEV of each increasing induction variable used by the
+ // select instructions is identical.
+ if (!OrigPhi->hasOneUse())
+ return InstDesc(false, I);
+
+ // TODO: Match selects with multi-use cmp conditions.
+ CmpInst::Predicate Pred;
+ Value *TrueVal, *FalseVal;
+ if (!match(I, m_Select(m_OneUse(m_Cmp(Pred, m_Value(), m_Value())),
+ m_Value(TrueVal), m_Value(FalseVal))))
+ return InstDesc(false, I);
+
+ Value *NonRdxPhi = nullptr;
+ if (OrigPhi == dyn_cast<PHINode>(TrueVal))
+ NonRdxPhi = FalseVal;
+ else if (OrigPhi == dyn_cast<PHINode>(FalseVal))
+ NonRdxPhi = TrueVal;
+ else
+ return InstDesc(false, I);
+
+ auto IsIncreasingLoopInduction = [&](Value *V) {
----------------
Mel-Chen wrote:
Your comment suggests removing the lambda function `IsIncreasingLoopInduction` and replacing it with a direct return statements, if I understand correctly.
This is indeed possible, but I worry that it could reduce the readability, making it difficult for developers to understand that this code section is confirming whether an induction variable is increasing.
Could you suggest a better approach that balances readability?
https://github.com/llvm/llvm-project/pull/67812
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