[clang] [time-trace] Add a new time trace scope variable named "ParseDeclarationOrFunctionDefinition". (PR #65268)

[via cfe-commits]

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@@ -1224,6 +1225,9 @@ Parser::DeclGroupPtrTy Parser::ParseDeclOrFunctionDefInternal(
 Parser::DeclGroupPtrTy Parser::ParseDeclarationOrFunctionDefinition(
     ParsedAttributes &Attrs, ParsedAttributes &DeclSpecAttrs,
     ParsingDeclSpec *DS, AccessSpecifier AS) {
+  // Add an enclosing time trace scope for a bunch of small scopes with
+  // "EvaluateAsConstExpr".
+  llvm::TimeTraceScope TimeScope("ParseDeclarationOrFunctionDefinition");
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MaggieYingYi wrote:

Hi @AaronBallman ,

Many thanks for your suggestions.

> Should we supply a second argument to the ctor here so that we can include the function name as part of the time trace output?

Good point. I cannot add `function name` as a second argument in the `Parser::ParseDeclarationOrFunctionDefinition()` since the function definition/declaration is not parsed yet. However, as you suggested, I have added a new time trace variable to `Parser::ParseFunctionDefinition()` and included the function name as a part of the time trace output in the commit https://github.com/llvm/llvm-project/pull/65268/commits/deecf8c2.

> Also, I wonder if we want more or less granularity here. This will fire for function declarations and function definitions, so we could add it to `Parser::ParseFunctionDefinition()` instead to only time function definition parsing, or we could split it up so that we time parsing function signatures separately from parsing the function body so we can more easily see how much time is spent in which activity.

As mentioned above, I have added a new time trace variable of `ParseFunctionDefinition`. However, I cannot remove the time trace variable of "ParseDeclarationOrFunctionDefinition" since `ParseFunctionDefinition` doesn't enclose small time scopes (named EvaluateAsConstExpr) in the function declarations. The initial issue is shown in the function declarations (not function definitions).

I will use a simple test to show the issue.

1. Source file test.cpp:
```
$ cat test.cpp
consteval double slow_func() {
    double d = 0.0;
    for (int i = 0; i < 100; ++i) {
        d += i;
    }
    return d;
}

void slow_test() {
    constexpr auto slow_value = slow_func();
}

int slow_arr[12 + 34 * 56 + static_cast<int>(slow_func())];
```

2. Compile test.cpp using the clang which is built using the repo of yingyi/main/time-trace:
```
$ clang.exe -ftime-trace -ftime-trace-granularity=0  -std=c++20 test.cpp -c -o test.o
```

3. View the generated time-trace file of test.json using `chrome://tracing/`.

For `slow_arr`, it uses the `slow_func()` declaration. `EvaluateAsConstantExpr` is under the time scope of `ParseDeclarationOrFunctionDefinition` but not under the time scope of `ParseFunctionDefinition`.

Therefore, I think we need both time scope variables of `ParseDeclarationOrFunctionDefinition` and 
`ParseFunctionDefinition`.

https://github.com/llvm/llvm-project/pull/65268