[SE-0454]: adopt mimalloc for Windows toolchain (#2618)

* proposals: propose adopting mimalloc for Windows toolchain

Using mimalloc for the memory allocator in the compiler yields ~4%
performance improvement according to local benchmarks. Propose this
allocator as the default for the Windows build.

* Allocate SE-0454 for the review.

* Update 0454-memory-allocator.md

Add a link to the build.ps1 change for the feature.

---------

Co-authored-by: Alastair Houghton <[email protected]>

Author: compnerd

proposals/0454-memory-allocator.md

@@ -0,0 +1,73 @@
+# Custom Allocator for Toolchain
+
+* Proposal: [SE-0454](0454-memory-allocator.md)
+* Authors: [Saleem Abdulrasool](https://github.com/compnerd)
+* Review Manager: [Alastair Houghton](https://github.com/al45tair)
+* Status: **Awaiting Review**
+* Vision: N/A
+* Roadmap: N/A
+* Bug: N/A
+* Implementation: [swiftlang/swift#76563](https://github.com/swiftlang/swift/pull/76563)
+* Upcoming Feature Flag: N/A
+* Previous Proposal: N/A
+* Previous Revision: N/A
+* Review: N/A
+
+## Introduction
+
+The tools in the Swift toolchain require allocating data structures for
+compiling the code. Different memory allocators have differing performance
+characteristics. Changing the default memory allocator away from the default
+(system) allocator can yield benefits if the allocator is better tuned to the
+allocation patterns of the compiler.
+
+## Motivation
+
+A more effecient memory allocator would improve the performance of the compiler
+on Windows. This allows better developer productivity by reducing compile time.
+
+## Proposed solution
+
+We propose to adopt mimalloc as the memory allocator for the Swift toolchain on
+Windows.
+
+## Detailed design
+
+Building a test codebase yielded a 4% build time decrease when the toolchain was
+built with mimalloc.
+
+## Source compatibility
+
+This proposal does not affect source compatibility.
+
+## ABI compatibility
+
+This proposal does not affect ABI of code.
+
+## Implications on adoption
+
+Additional files will need to be built, packaged, and shipped as part of the
+toolchain. The mimalloc build is relatively light and the overall build time
+impact is minimal.
+
+This change has no implications for the runtime, only the toolchain is changed.
+
+## Future directions
+
+None at this time.
+
+## Alternatives considered
+
+Alternative memory allocators were considered, including
+[tcmalloc](https://github.com/google/tcmalloc) and
+[tbb](https://github.com/intel/tbb). mimalloc is well supported, developed by
+Microsoft, and has better characteristics comparatively.
+
+Leaving the allocator on the default system allocator leaves the compiler
+without the performance improvements of an alternative allocator.
+
+## Acknowledgements
+
+Special thanks to @hjyamauchi for performing the work to integrate the mimalloc
+build into the Windows build and collecting the performance numbers that showed
+the improvement.