[clang-format] @lefticus just taught the world how to use [[unlikely]] but we forgot...

Summary:
https://twitter.com/lefticus/status/1262392152950288384?s=20

Jason Turner's (@lefticus) most recent C++ weekly explains the usage of [[likely]] and [[unlikely]] in an 'if/else' context in C++ 20

clang-format leaves the code a little messy afterwards..

```
if (argc > 5)
  [[unlikely]] {
    // ...
  }
else if (argc < 0)
  [[likely]] {
    // ...
  }
else
  [[likely]] {
    // ...
  }
```

try to improve the situation

```
if (argc > 5) [[unlikely]] {
  // ...
} else if (argc < 0) [[likely]] {
  // ...
} else [[likely]] {
  // ...
}
```

Reviewed By: JakeMerdichAMD

Subscribers: cfe-commits, lefticus

Tags: #clang, #clang-format

Differential Revision: https://reviews.llvm.org/D80144

diff --git a/clang/lib/Format/UnwrappedLineParser.cpp b/clang/lib/Format/UnwrappedLineParser.cpp
index de820ba..7aaf634 100644 (file)
--- a/clang/lib/Format/UnwrappedLineParser.cpp
+++ b/clang/lib/Format/UnwrappedLineParser.cpp
@@ -1956,6 +1956,9 @@ void UnwrappedLineParser::parseIfThenElse() {
     nextToken();
   if (FormatTok->Tok.is(tok::l_paren))
     parseParens();
+  // handle [[likely]] / [[unlikely]]
+  if (FormatTok->is(tok::l_square))
+    parseSquare();
   bool NeedsUnwrappedLine = false;
   if (FormatTok->Tok.is(tok::l_brace)) {
     CompoundStatementIndenter Indenter(this, Style, Line->Level);
@@ -1972,6 +1975,9 @@ void UnwrappedLineParser::parseIfThenElse() {
   }
   if (FormatTok->Tok.is(tok::kw_else)) {
     nextToken();
+    // handle [[likely]] / [[unlikely]]
+    if (FormatTok->is(tok::l_square))
+      parseSquare();
     if (FormatTok->Tok.is(tok::l_brace)) {
       CompoundStatementIndenter Indenter(this, Style, Line->Level);
       parseBlock(/*MustBeDeclaration=*/false);

diff --git a/clang/unittests/Format/FormatTest.cpp b/clang/unittests/Format/FormatTest.cpp
index 35da582..a0b1dc7 100644 (file)
--- a/clang/unittests/Format/FormatTest.cpp
+++ b/clang/unittests/Format/FormatTest.cpp
@@ -16363,6 +16363,36 @@ TEST_F(FormatTest, VeryLongNamespaceCommentSplit) {
              Style));
 }
 
+TEST_F(FormatTest, LikelyUnlikely) {
+  FormatStyle Style = getLLVMStyle();
+
+  verifyFormat("if (argc > 5) [[unlikely]] {\n"
+               "  return 29;\n"
+               "}",
+               Style);
+
+  verifyFormat("if (argc > 5) [[likely]] {\n"
+               "  return 29;\n"
+               "}",
+               Style);
+
+  verifyFormat("if (argc > 5) [[unlikely]] {\n"
+               "  return 29;\n"
+               "} else [[likely]] {\n"
+               "  return 42;\n"
+               "}\n",
+               Style);
+
+  verifyFormat("if (argc > 5) [[unlikely]] {\n"
+               "  return 29;\n"
+               "} else if (argc > 10) [[likely]] {\n"
+               "  return 99;\n"
+               "} else {\n"
+               "  return 42;\n"
+               "}\n",
+               Style);
+}
+
 } // namespace
 } // namespace format
 } // namespace clang