Merge pull request swiftlang#86784 from eeckstein/temp-rvalue-elimination-6.3

[6.3] TempRValueElimination: fix a problem with `load`-`store` copy elimination

Author: eeckstein

SwiftCompilerSources/Sources/Optimizer/FunctionPasses/TempRValueElimination.swift

@@ -430,10 +430,15 @@ private func getLastUseWhileSourceIsNotModified(of copy: CopyLikeInstruction,
                                                 uses: InstructionSetWithCount,
                                                 _ context: FunctionPassContext) -> Instruction?
 {
-  if uses.isEmpty {
+  var numUsesFound = 0
+  if copy == copy.loadingInstruction {
+    // As we are starting the iteration at the next instruction after the copy's load instruction
+    // we pretend to having seen the copy instruction already.
+    numUsesFound += 1
+  }
+  if uses.count == numUsesFound {
     return copy
   }
-  var numUsesFound = 0
   let aliasAnalysis = context.aliasAnalysis
 
   // We already checked that the useful lifetime of the `alloc_stack` ends in the same block as the `copy`.
@@ -484,7 +489,7 @@ private struct UseCollector : AddressDefUseWalker {
     for use in allocStack.uses {
       switch use.instruction {
       case copy:
-        break
+        uses.insert(copy)
       case is DeallocStackInst:
         // Deallocations are allowed to be in a different block.
         break

test/SILOptimizer/temp_rvalue_opt_ossa.sil

@@ -1790,3 +1790,22 @@ bb2:
   return %9
 }
 
+// CHECK-LABEL: sil [ossa] @dealloc_stack_between_load_and_store :
+// CHECK:         %4 = load [take] %2
+// CHECK-NEXT:    dealloc_stack %2
+// CHECK-NEXT:    store %4 to [init] %1
+// CHECK-LABEL: } // end sil function 'dealloc_stack_between_load_and_store'
+sil [ossa] @dealloc_stack_between_load_and_store : $@convention(thin) (@owned AnyObject) -> () {
+bb0(%0 : @owned $AnyObject):
+  %1 = alloc_stack $AnyObject
+  %2 = alloc_stack [lexical] $AnyObject
+  store %0 to [init] %2
+  %4 = load [take] %2
+  dealloc_stack %2
+  store %4 to [init] %1
+  destroy_addr %1
+  dealloc_stack %1
+  %9 = tuple ()
+  return %9
+}
+