[llvm][mustache] Optimize accessor splitting with a single pass

[ilovepi]

The splitMustacheString function previously used a loop of
StringRef::split and StringRef::trim. This was inefficient as
it scanned each segment of the accessor string multiple times.

This change introduces a custom splitAndTrim function that
performs both operations in a single pass over the string,
reducing redundant work and improving performance, most notably
in the number of CPU cycles executed.

Metric	Baseline	Optimized	Change
Time (ms)	35.57	35.36	-0.59%
Cycles	34.91M	34.26M	-1.86%
Instructions	85.54M	85.24M	-0.35%
Branch Misses	111.9K	112.2K	+0.27%
Cache Misses	242.1K	239.9K	-0.91%

[ilovepi]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• [llvm][mustache] Avoid extra allocations in parseSection #159199
• [llvm][mustache] Optimize accessor splitting with a single pass #159198 👈 (View in Graphite)
• [llvm][mustache] Avoid redundant saves in accessor splitting #159197
• [llvm][mustache] Use single pass when tokenizing #159196
• [llvm][mustache] Avoid extra copy for json strings #159195
• [llvm][mustache] Use BumpPtrAllocator to save ASTNodes #159194
• [llvm][mustache] Simplify debug logging #159193
• [llvm][mustache] Fix failing StandaloneIndentation test #159192
• [llvm][mustache] Introduce MustacheContext to simplify mustache APIs #159191
• [llvm][mustache] Remove out parameters from processTags() #159190
• [llvm][mustache] Refactor template rendering #159189
• [llvm][mustache] Refactor tokenizer for clarity #159188
• [llvm][mustache] Support setting delimiters in templates #159187
• [llvm][mustache] Precommit test for Set Delimiter #159186
• [llvm][mustache] Align standalone partial indentation with spec #159185
• [llvm][mustache] Precommit test for StandaloneIndentation #159184
• [llvm][mustache] Add support for Triple Mustache #159183
• [llvm][mustache] Precommit tests for Triple Mustache #159182
• main

This stack of pull requests is managed by Graphite. Learn more about stacking.

[llvmbot]

@llvm/pr-subscribers-llvm-support

Author: Paul Kirth (ilovepi)

Changes

The splitMustacheString function previously used a loop of
StringRef::split and StringRef::trim. This was inefficient as
it scanned each segment of the accessor string multiple times.

This change introduces a custom splitAndTrim function that
performs both operations in a single pass over the string,
reducing redundant work and improving performance, most notably
in the number of CPU cycles executed.

Metric	Baseline	Optimized	Change
Time (ms)	35.57	35.36	-0.59%
Cycles	34.91M	34.26M	-1.86%
Instructions	85.54M	85.24M	-0.35%
Branch Misses	111.9K	112.2K	+0.27%
Cache Misses	242.1K	239.9K	-0.91%

---

Full diff: https://github.com/llvm/llvm-project/pull/159198.diff

1 Files Affected:

• (modified) llvm/lib/Support/Mustache.cpp (+27-7)

diff --git a/llvm/lib/Support/Mustache.cpp b/llvm/lib/Support/Mustache.cpp
index fcb55c4edd815..8139e9eb4717f 100644
--- a/llvm/lib/Support/Mustache.cpp
+++ b/llvm/lib/Support/Mustache.cpp
@@ -35,6 +35,32 @@ static bool isContextFalsey(const json::Value *V) {
   return isFalsey(*V);
 }
 
+static void splitAndTrim(StringRef Str, SmallVectorImpl<StringRef> &Tokens) {
+  size_t CurrentPos = 0;
+  while (CurrentPos < Str.size()) {
+    // Find the next delimiter.
+    size_t DelimiterPos = Str.find('.', CurrentPos);
+
+    // If no delimiter is found, process the rest of the string.
+    if (DelimiterPos == StringRef::npos) {
+      DelimiterPos = Str.size();
+    }
+
+    // Get the current part, which may have whitespace.
+    StringRef Part = Str.slice(CurrentPos, DelimiterPos);
+
+    // Manually trim the part without creating a new string object.
+    size_t Start = Part.find_first_not_of(" \t\r\n");
+    if (Start != StringRef::npos) {
+      size_t End = Part.find_last_not_of(" \t\r\n");
+      Tokens.push_back(Part.slice(Start, End + 1));
+    }
+
+    // Move past the delimiter for the next iteration.
+    CurrentPos = DelimiterPos + 1;
+  }
+}
+
 static Accessor splitMustacheString(StringRef Str, MustacheContext &Ctx) {
   // We split the mustache string into an accessor.
   // For example:
@@ -47,13 +73,7 @@ static Accessor splitMustacheString(StringRef Str, MustacheContext &Ctx) {
     // It's a literal, so it doesn't need to be saved.
     Tokens.push_back(".");
   } else {
-    while (!Str.empty()) {
-      StringRef Part;
-      std::tie(Part, Str) = Str.split('.');
-      // Each part of the accessor needs to be saved to the arena
-      // to ensure it has a stable address.
-      Tokens.push_back(Part.trim());
-    }
+    splitAndTrim(Str, Tokens);
   }
   // Now, allocate memory for the array of StringRefs in the arena.
   StringRef *ArenaTokens = Ctx.Allocator.Allocate<StringRef>(Tokens.size());

[evelez7]

LGTM w/ nit

[evelez7]

nit: can omit the curly braces here