[SILOpt][OSLog] Skip constant folding StaticString in the

OSLogOptimization pass

The OSLogOptimization pass constant evaluates and folds SIL instructions
that are inlined from the construction of the string interpolations passed
to the log calls, which enables replacing the dynamic format string construction
with a static format string. In addition to folding constant strings, it also folds
constant integers and arrays whose elements are constants. This change makes it
skip folding static strings, since they are already efficiently represented.

rdar://146028438

Author: ravikandhadai

lib/SILOptimizer/Mandatory/OSLogOptimization.cpp

@@ -938,6 +938,16 @@ static void substituteConstants(FoldState &foldState) {
   for (SILValue constantSILValue : foldState.getConstantSILValues()) {
     SymbolicValue constantSymbolicVal =
         evaluator.lookupConstValue(constantSILValue).value();
+    CanType instType = constantSILValue->getType().getASTType();
+
+    // If the SymbolicValue is a string but the instruction that is folded is
+    // not String typed, we are tracking a StaticString which is represented as
+    // a raw pointer. Skip folding StaticString as they are already efficiently
+    // represented.
+    if (constantSymbolicVal.getKind() == SymbolicValue::String &&
+        !instType->isString())
+      continue;
+
     // Make sure that the symbolic value tracked in the foldState is a constant.
     // In the case of ArraySymbolicValue, the array storage could be a non-constant
     // if some instruction in the array initialization sequence was not evaluated
@@ -976,7 +986,6 @@ static void substituteConstants(FoldState &foldState) {
 
     SILBuilderWithScope builder(insertionPoint);
     SILLocation loc = insertionPoint->getLoc();
-    CanType instType = constantSILValue->getType().getASTType();
     SILValue foldedSILVal = emitCodeForSymbolicValue(
         constantSymbolicVal, instType, builder, loc, foldState.stringInfo);