[llvm][mustache] Avoid redundant saves in accessor splitting

[ilovepi]

The splitMustacheString function was saving StringRefs that
were already backed by an arena-allocated string. This was
unnecessary work. This change removes the redundant
Ctx.Saver.save() call.

This optimization provides a small but measurable performance
improvement on top of the single-pass tokenizer, most notably
reducing branch misses.

Metric	Baseline	Optimized	Change
Time (ms)	35.77	35.57	-0.56%
Cycles	35.16M	34.91M	-0.71%
Instructions	85.77M	85.54M	-0.27%
Branch Misses	113.9K	111.9K	-1.76%
Cache Misses	237.7K	242.1K	+1.85%

[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
• [llvm][mustache] Avoid redundant saves in accessor splitting #159197 👈 (View in Graphite)
• [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

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[llvmbot]

@llvm/pr-subscribers-llvm-support

Author: Paul Kirth (ilovepi)

Changes

The splitMustacheString function was saving StringRefs that
were already backed by an arena-allocated string. This was
unnecessary work. This change removes the redundant
Ctx.Saver.save() call.

This optimization provides a small but measurable performance
improvement on top of the single-pass tokenizer, most notably
reducing branch misses.

Metric	Baseline	Optimized	Change
Time (ms)	35.77	35.57	-0.56%
Cycles	35.16M	34.91M	-0.71%
Instructions	85.77M	85.54M	-0.27%
Branch Misses	113.9K	111.9K	-1.76%
Cache Misses	237.7K	242.1K	+1.85%

---

Full diff: https://github.com/llvm/llvm-project/pull/159197.diff

1 Files Affected:

• (modified) llvm/lib/Support/Mustache.cpp (+1-1)

diff --git a/llvm/lib/Support/Mustache.cpp b/llvm/lib/Support/Mustache.cpp
index 63798c50f57ee..fcb55c4edd815 100644
--- a/llvm/lib/Support/Mustache.cpp
+++ b/llvm/lib/Support/Mustache.cpp
@@ -52,7 +52,7 @@ static Accessor splitMustacheString(StringRef Str, MustacheContext &Ctx) {
       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(Ctx.Saver.save(Part.trim()));
+      Tokens.push_back(Part.trim());
     }
   }
   // Now, allocate memory for the array of StringRefs in the arena.