[Mlir-commits] [mlir] [MLIR][SCFToOpenMP] Add automatic-num-threads option (PR #85771)

[Kiran Chandramohan]

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@@ -437,7 +440,37 @@ struct ParallelOpLowering : public OpRewritePattern<scf::ParallelOp> {
     rewriter.eraseOp(reduce);
 
     Value numThreadsVar;
-    if (numThreads > 0) {
+    if (automaticNumThreads) {
+      unsigned inferredNumThreads = 1;
+      for (auto [lb, ub, step] :
+           llvm::zip_equal(parallelOp.getLowerBound(),
+                           parallelOp.getUpperBound(), parallelOp.getStep())) {
+        std::optional<int64_t> cstLb = getConstantIntValue(lb);
+        std::optional<int64_t> cstUb = getConstantIntValue(ub);
+        std::optional<int64_t> cstStep = getConstantIntValue(step);
+
+        if (!cstLb.has_value())
+          return emitError(loc)
+                 << "Expected a parallel loop with constant lower bounds when "
+                    "trying to automatically infer number of threads";
+
+        if (!cstUb.has_value())
+          return emitError(loc)
+                 << "Expected a parallel loop with constant upper bounds when "
+                    "trying to automatically infer number of threads\n";
+
+        if (!cstStep.has_value())
+          return emitError(loc)
+                 << "Expected a forall with constant steps when trying to "
+                    "automatically infer number of threads\n";
+
+        inferredNumThreads =
+            inferredNumThreads *
+            ((cstUb.value() - cstLb.value()) / cstStep.value());
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kiranchandramohan wrote:

Wouldn't this lead to too many threads, i.e equal to 1 thread per iteration?

https://github.com/llvm/llvm-project/pull/85771