Allow direct comparison of ConstString against StringRef

Summary:
When we want to compare a ConstString against a string literal (or any other non-ConstString),
we currently have to explicitly turn the other string into a ConstString. This makes sense as
comparing ConstStrings against each other is only a fast pointer comparison.

However, currently we (rather incorrectly) use in several places in LLDB temporary ConstStrings when
we just want to compare a given ConstString against a hardcoded value, for example like this:
```
if (extension != ConstString(".oat") && extension != ConstString(".odex"))
```

Obviously this kind of defeats the point of ConstStrings. In the comparison above we would
construct two temporary ConstStrings every time we hit the given code. Constructing a
ConstString is relatively expensive: we need to go to the StringPool, take a read and possibly
an exclusive write-lock and then look up our temporary string in the string map of the pool.
So we do a lot of heavy work for essentially just comparing a <6 characters in two strings.

I initially wanted to just fix these issues by turning the temporary ConstString in static variables/
members, but that made the code much less readable. Instead I propose to add a new overload
for the ConstString comparison operator that takes a StringRef. This comparison operator directly
compares the ConstString content against the given StringRef without turning the StringRef into
a ConstString.

This means that the example above can look like this now:
```
if (extension != ".oat" && extension != ".odex")
```
It also no longer has to unlock/lock two locks and call multiple functions in other TUs for constructing
the temporary ConstString instances. Instead this should end up just being a direct string comparison
of the two given strings on most compilers.

This patch also directly updates all uses of temporary and short ConstStrings in LLDB to use this new
comparison operator. It also adds a some unit tests for the new and old comparison operator.

Reviewers: #lldb, JDevlieghere, espindola, amccarth

Reviewed By: JDevlieghere, amccarth

Subscribers: amccarth, clayborg, JDevlieghere, emaste, arichardson, MaskRay, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D60667

llvm-svn: 359281

1 files changed, 4 insertions, 4 deletions

diff --git a/lldb/source/Plugins/Language/CPlusPlus/LibStdcpp.cpp b/lldb/source/Plugins/Language/CPlusPlus/LibStdcpp.cpp
index 0aac93c3c76..0e0f6663c92 100644
--- a/lldb/source/Plugins/Language/CPlusPlus/LibStdcpp.cpp
+++ b/lldb/source/Plugins/Language/CPlusPlus/LibStdcpp.cpp
@@ -142,9 +142,9 @@ bool LibstdcppMapIteratorSyntheticFrontEnd::MightHaveChildren() { return true; }
 
 size_t LibstdcppMapIteratorSyntheticFrontEnd::GetIndexOfChildWithName(
     ConstString name) {
-  if (name == ConstString("first"))
+  if (name == "first")
     return 0;
-  if (name == ConstString("second"))
+  if (name == "second")
     return 1;
   return UINT32_MAX;
 }
@@ -224,7 +224,7 @@ bool VectorIteratorSyntheticFrontEnd::MightHaveChildren() { return true; }
 
 size_t VectorIteratorSyntheticFrontEnd::GetIndexOfChildWithName(
     ConstString name) {
-  if (name == ConstString("item"))
+  if (name == "item")
     return 0;
   return UINT32_MAX;
 }
@@ -374,7 +374,7 @@ bool LibStdcppSharedPtrSyntheticFrontEnd::MightHaveChildren() { return true; }
 
 size_t LibStdcppSharedPtrSyntheticFrontEnd::GetIndexOfChildWithName(
     ConstString name) {
-  if (name == ConstString("_M_ptr"))
+  if (name == "_M_ptr")
     return 0;
   return UINT32_MAX;
 }