avdlee__xcode-build-optimization-agent-skill/references/build-settings-best-practices.md

Build Settings Best Practices

This reference lists Xcode build settings that affect build performance. Use it to audit a project and produce a pass/fail checklist.

The scope is strictly build performance. Do not flag language-migration settings like SWIFT_STRICT_CONCURRENCY or SWIFT_UPCOMING_FEATURE_* -- those are developer adoption choices unrelated to build speed.

How To Read This Reference

Each setting includes:

• Setting name and the Xcode build-settings key
• Recommended value for Debug and Release
• Why it matters for build time
• Risk of changing it

Use checkmark and cross indicators when reporting:

• [x] -- setting matches the recommended value
• [ ] -- setting does not match; include the actual value and the expected value

Debug Configuration

These settings optimize for fast iteration during development.

Compilation Mode

• Key: SWIFT_COMPILATION_MODE
• Recommended: singlefile (Xcode UI: "Incremental"; or unset -- Xcode defaults to singlefile for Debug)
• Why: Single-file mode recompiles only changed files. wholemodule recompiles the entire target on every change.
• Risk: Low

Swift Optimization Level

• Key: SWIFT_OPTIMIZATION_LEVEL
• Recommended: -Onone
• Why: Optimization passes add significant compile time. Debug builds do not benefit from runtime speed improvements.
• Risk: Low

GCC Optimization Level

• Key: GCC_OPTIMIZATION_LEVEL
• Recommended: 0
• Why: Same rationale as Swift optimization level, but for C/C++/Objective-C sources.
• Risk: Low

Build Active Architecture Only

• Key: ONLY_ACTIVE_ARCH (BUILD_ACTIVE_ARCHITECTURE_ONLY)
• Recommended: YES
• Why: Building all architectures doubles or triples compile and link time for no debug benefit.
• Risk: Low

Debug Information Format

• Key: DEBUG_INFORMATION_FORMAT
• Recommended: dwarf
• Why: dwarf-with-dsym generates a separate dSYM bundle which adds overhead. Plain dwarf embeds debug info directly in the binary, which is sufficient for local debugging.
• Risk: Low

Enable Testability

• Key: ENABLE_TESTABILITY
• Recommended: YES
• Why: Required for @testable import. Adds minor overhead by exporting internal symbols, but this is expected during development.
• Risk: Low

Active Compilation Conditions

• Key: SWIFT_ACTIVE_COMPILATION_CONDITIONS
• Recommended: Should include DEBUG
• Why: Guards conditional compilation blocks (e.g., #if DEBUG) and ensures debug-only code paths are included.
• Risk: Low

Eager Linking

• Key: EAGER_LINKING
• Recommended: YES
• Why: Allows the linker to start work before all compilation tasks finish, reducing wall-clock build time. Particularly effective for Debug builds where link time is a meaningful fraction of total build time.
• Risk: Low

Release Configuration

These settings optimize for production builds.

Compilation Mode

• Key: SWIFT_COMPILATION_MODE
• Recommended: wholemodule
• Why: Whole-module optimization produces faster runtime code. Build time is secondary for release.
• Risk: Low

Swift Optimization Level

• Key: SWIFT_OPTIMIZATION_LEVEL
• Recommended: -O or -Osize
• Why: Produces optimized binaries. -Osize trades some speed for smaller binary size.
• Risk: Low

GCC Optimization Level

• Key: GCC_OPTIMIZATION_LEVEL
• Recommended: s
• Why: Optimizes C/C++/Objective-C for size, matching the typical release expectation.
• Risk: Low

Build Active Architecture Only

• Key: ONLY_ACTIVE_ARCH
• Recommended: NO
• Why: Release builds must include all supported architectures for distribution.
• Risk: Low

Debug Information Format

• Key: DEBUG_INFORMATION_FORMAT
• Recommended: dwarf-with-dsym
• Why: dSYM bundles are required for crash symbolication in production.
• Risk: Low

Enable Testability

• Key: ENABLE_TESTABILITY
• Recommended: NO
• Why: Removes internal-symbol export overhead from release builds. Testing should use Debug configuration.
• Risk: Low

General (All Configurations)

Compilation Caching

• Key: COMPILATION_CACHING
• Recommended: YES
• Why: Caches compilation results for Swift and C-family sources so repeated compilations of the same inputs are served from cache. The biggest wins come from branch switching and clean builds where source files are recompiled unchanged. This is an opt-in feature. The umbrella setting controls both SWIFT_ENABLE_COMPILE_CACHE and CLANG_ENABLE_COMPILE_CACHE under the hood; those can be toggled independently if needed.
• Measurement: The benchmark script auto-detects this setting and runs a cached clean phase that measures clean builds with a warm compilation cache. Standard clean builds may show overhead from cache population; the cached clean metric captures the realistic developer benefit.
• Risk: Low -- can also be enabled via per-user project settings so it does not need to be committed to the shared project file.

Integrated Swift Driver

• Key: SWIFT_USE_INTEGRATED_DRIVER
• Recommended: YES
• Why: Uses the integrated Swift driver which runs inside the build system process, eliminating inter-process overhead for compilation scheduling. Enabled by default in modern Xcode but worth verifying in migrated projects.
• Risk: Low

Clang Module Compilation

• Key: CLANG_ENABLE_MODULES
• Recommended: YES
• Why: Enables Clang module compilation for C/Objective-C sources, caching module maps on disk instead of reprocessing headers on every import. Eliminates redundant header parsing across translation units.
• Risk: Low

Explicit Module Builds

• Key: SWIFT_ENABLE_EXPLICIT_MODULES (C/ObjC enabled by default in Xcode 16+; for Swift use _EXPERIMENTAL_SWIFT_EXPLICIT_MODULES)
• Recommended: Evaluate per-project
• Why: Makes module compilation visible to the build system as discrete tasks, improving parallelism and scheduling. Reduces redundant module rebuilds by making dependency edges explicit. Some projects see regressions due to the overhead of dependency scanning, so benchmark before and after enabling.
• Risk: Medium -- test thoroughly; currently experimental for Swift targets.

Cross-Target Consistency

These checks find settings differences between targets that cause redundant build work.

Project-Level vs Target-Level Overrides

Build-affecting settings should be set at the project level unless a target has a specific reason to override. Unnecessary per-target overrides cause confusion and can silently create module variants.

Settings to check for project-level consistency:

• SWIFT_COMPILATION_MODE
• SWIFT_OPTIMIZATION_LEVEL
• ONLY_ACTIVE_ARCH
• DEBUG_INFORMATION_FORMAT

Module Variant Duplication

When multiple targets import the same SPM package but compile with different Swift compiler options, the build system produces separate module variants for each combination. This inflates SwiftEmitModule task counts.

Check for drift in:

• SWIFT_OPTIMIZATION_LEVEL
• SWIFT_COMPILATION_MODE
• OTHER_SWIFT_FLAGS
• Target-level build settings that override project defaults

Out of Scope

Do not flag the following as build-performance issues:

• SWIFT_STRICT_CONCURRENCY -- language migration choice
• SWIFT_UPCOMING_FEATURE_* -- language migration choice
• SWIFT_APPROACHABLE_CONCURRENCY -- language migration choice
• SWIFT_ACTIVE_COMPILATION_CONDITIONS values beyond DEBUG (e.g., WIDGETS, APPCLIP) -- intentional per-target customization

Checklist Output Format

When reporting results, use this structure:

### Debug Configuration
- [x] `SWIFT_COMPILATION_MODE`: `singlefile` (recommended: `singlefile`)
- [ ] `DEBUG_INFORMATION_FORMAT`: `dwarf-with-dsym` (recommended: `dwarf`)
- [x] `SWIFT_OPTIMIZATION_LEVEL`: `-Onone` (recommended: `-Onone`)
...

### Release Configuration
- [x] `SWIFT_COMPILATION_MODE`: `wholemodule` (recommended: `wholemodule`)
...

### General (All Configurations)
- [ ] `COMPILATION_CACHING`: `NO` (recommended: `YES`)
...

### Cross-Target Consistency
- [x] All targets inherit `SWIFT_OPTIMIZATION_LEVEL` from project level
- [ ] `OTHER_SWIFT_FLAGS` differs between Stock Analyzer and StockAnalyzerClip
...