llvm/llvm-project

performance optimization: special-case deduction for variables of type `auto`

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#186274 opened on Mar 12, 2026

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c++11clang:frontendenhancementgood first issuequality-of-implementationslow-compile

Description

General-purpose auto deduction involves some heavy machinery: we synthesize a template, perform a substitution on the type, perform deduction against the substituted type, perform another substitution, and so on. All of that adds up to a significant fraction of the total compile time if auto is used heavily.

But in places where auto is used heavily, the complete type is very frequently simply auto (or const auto), in which case we don't actually need any of that heavy machinery. Clang should detect the special case where the type is just auto (Context.AutoDeductTy) and the initializer is not an InitListExpr, and compute the decayed type of the initializer and directly build the deduced type.

$ for i in $(seq 1000000); do echo "auto v$i = 0;"; done >! ~/auto.cpp
$ for i in $(seq 1000000); do echo "int v$i = 0;"; done >! ~/int.cpp  
$ time clang++ ~/auto.cpp -fsyntax-only                               
clang++ ~/auto.cpp -fsyntax-only  6.57s user 0.36s system 99% cpu 6.935 total
$ time clang++ ~/int.cpp -fsyntax-only                                
clang++ ~/int.cpp -fsyntax-only  4.40s user 0.22s system 99% cpu 4.629 total

Contributor guide

Tech stack
cpp
Domain
backendperformance
Issue type
performance
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
5
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
over 1 week
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
C++ template deductionClang development setupcompiler internals
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
15
Research direction
Investigate the file clang/lib/Sema/SemaTemplateDeduction.cpp around line 5193 to understand the current deduction flow. The issue suggests adding a special case for when the declared type is simply `auto` (Context.AutoDeductTy) and the initializer is not an InitListExpr. Then implement a path that directly computes the decayed type of the initializer to build the deduced type, bypassing the heavy machinery. Test with the provided benchmark to confirm performance improvement.