[llvm] [CodeGen][MISched] Handle empty sized resource usage. (PR #75951)

[Francesco Petrogalli via llvm-commits]

================
@@ -4266,6 +4266,12 @@ unsigned ResourceSegments::getFirstAvailableAt(
   assert(std::is_sorted(std::begin(_Intervals), std::end(_Intervals),
                         sortIntervals) &&
          "Cannot execute on an un-sorted set of intervals.");
+
+  // Zero resource usage is allowed by TargetSchedule.td but we do not construct
+  // a ResourceSegment interval for that situation.
+  if (AcquireAtCycle == Cycle)
+    return Cycle;
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fpetrogalli wrote:

This seems to be wrong. Say that `CurrCycle` is 200, and that ResourceX have a `[AcquireAtCycle,ReleaseAtCycle]` pair `[5,5]` (note that `Cycle == ReleaseAtCycle` in the naming of the method's parameters... unfortunate)

Your code will say that the resource is available at cycle 5 (not even considering if the resource is busy at cycle 200), but in reality we can only schedule from cycle 200 onwards.

https://github.com/llvm/llvm-project/pull/75951