SILCombine: remove dead string.init_empty_with_capacity semantic calls

This avoids leftover runtime calls for constant folded large interpolated strings.

rdar://127308237

Author: eeckstein

lib/SILOptimizer/SILCombiner/SILCombinerApplyVisitors.cpp

@@ -1448,6 +1448,16 @@ static bool shouldReplaceCallByMetadataConstructor(CanType storageMetaTy) {
   return false;
 }
 
+static bool canBeRemovedIfResultIsNotUsed(SILFunction *f) {
+  if (f->getEffectsKind() < EffectsKind::ReleaseNone)
+    return true;
+
+  if (f->hasSemanticsAttr("string.init_empty_with_capacity"))
+    return true;
+
+  return false;
+}
+
 SILInstruction *SILCombiner::visitApplyInst(ApplyInst *AI) {
   Builder.setCurrentDebugScope(AI->getDebugScope());
   // apply{partial_apply(x,y)}(z) -> apply(z,x,y) is triggered
@@ -1469,7 +1479,7 @@ SILInstruction *SILCombiner::visitApplyInst(ApplyInst *AI) {
 
   // Optimize readonly functions with no meaningful users.
   SILFunction *SF = AI->getReferencedFunctionOrNull();
-  if (SF && SF->getEffectsKind() < EffectsKind::ReleaseNone) {
+  if (SF && canBeRemovedIfResultIsNotUsed(SF)) {
     UserListTy Users;
     if (recursivelyCollectARCUsers(Users, AI)) {
       if (eraseApply(AI, Users))

test/SILOptimizer/string_optimization.swift

@@ -69,6 +69,15 @@ public func testFoldConcat() -> String {
   return "a" + "b" + "c"
 }
 
+// CHECK-LABEL: sil hidden [noinline] @$s4test0A25InterpolationInLongStringSSyF :
+// CHECK-NOT: apply
+// CHECK-NOT: bb1
+// CHECK: } // end sil function '$s4test0A25InterpolationInLongStringSSyF'
+@inline(never)
+func testInterpolationInLongString() -> String {
+  return "\(#function) used in a veeeeeeeeeeeeeeeeeeeery long string"
+}
+
 // CHECK-LABEL: sil [noinline] @$s4test0A19UnqualifiedTypeNameSSyF 
 // CHECK-NOT: apply
 // CHECK-NOT: bb1
@@ -141,6 +150,9 @@ printEmbedded(testFoldStaticLet())
 // CHECK-OUTPUT: <abc>
 printEmbedded(testFoldConcat())
 
+// CHECK-OUTPUT: <testInterpolationInLongString() used in a veeeeeeeeeeeeeeeeeeeery long string>
+printEmbedded(testInterpolationInLongString())
+
 // CHECK-OUTPUT: <Inner>
 printEmbedded(testUnqualifiedTypeName())