Docs for specific coalescing. (#5886)

Add documentation describing the problem and algorithm for coalescing
the LLVM functions generated from Carbon generic functions into fewer
LLVM functions, where the LLVM types permit it.

Author: alinas

toolchain/docs/coalesce_generic_lowering.md

@@ -0,0 +1,292 @@
+# Coalescing generic functions emitted when lowering to LLVM IR
+
+<!--
+Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+Exceptions. See /LICENSE for license information.
+SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+-->
+
+<!-- toc -->
+
+## Table of contents
+
+-   [Overview](#overview)
+-   [Design details](#design-details)
+    -   [SemIR representation and why to coalesce during lowering](#semir-representation-and-why-to-coalesce-during-lowering)
+    -   [Recursion and strongly connected components (SCCs)](#recursion-and-strongly-connected-components-sccs)
+    -   [Function fingerprints](#function-fingerprints)
+    -   [Canonical specific to use](#canonical-specific-to-use)
+-   [Algorithm details](#algorithm-details)
+-   [Alternatives considered](#alternatives-considered)
+    -   [Coalescing in the front-end vs back-end?](#coalescing-in-the-front-end-vs-back-end)
+    -   [When to do coalescing in the front-end?](#when-to-do-coalescing-in-the-front-end)
+    -   [Compile-time trade-offs](#compile-time-trade-offs)
+    -   [Coalescing duplicate non-specific functions](#coalescing-duplicate-non-specific-functions)
+-   [Opportunities for further improvement](#opportunities-for-further-improvement)
+
+<!-- tocstop -->
+
+## Overview
+
+When lowering Carbon generics to LLVM, it is possible we emit duplicate LLVM IR
+functions. This document describes the algorithm implemented in
+[lowering](lower.md) for determining when and which generated specifics, while
+different at the Carbon language level, can be coalesced into a single one when
+lowering Carbon’s intermediate representation (_SemIR_), to
+[LLVM IR](https://llvm.org/docs/LangRef.html).
+
+The overall goal of this optimization is to avoid generating duplicate LLVM IR
+code where it is easy to determine this from the front-end. Such an optimization
+needs to be done after specialization, but there is some flexibility in when to
+do it afterwards: before lowering, through analysis of SemIR or during/after
+lowering.
+
+The goal of this doc is to describe the algorithm implemented in
+[specifics_coalescer](/toolchain/lower/specific_coalescer.h), from putting it
+into context, to the overall goal, the challenges and where there is still room
+for improvement in subsequent iterations.
+
+Determining the impact on compile-time is beyond the scope of this document, but
+an important problem to follow up on.
+
+## Design details
+
+In order to determine if two specific functions are equivalent, and a single one
+of them can be used instead of the other, the following need to be considered as
+part of the algorithm and its implementation.
+
+### SemIR representation and why to coalesce during lowering
+
+In SemIR, a specific function is defined by an unique tuple:
+`(function_id, specific_id)`. There is a single in-memory representation of a
+generic function’s body (not one for each specific), where the instructions that
+are different between specifics can be determined, on-demand, based on a given
+`specific_id`. Hence, determining if two specifics are equivalent needs to
+analyze if these specific-dependent instructions are equivalent at the LLVM IR
+level. This can only be determined after the eval phase is complete and using
+information on how Carbon types map to `llvm::Type`s.
+
+The algorithm described below does coalescing of specifics during lowering. Also
+see [alternatives considered](#alternatives-considered).
+
+### Recursion and strongly connected components (SCCs)
+
+Comparing if two different specific functions contain (access, invoke, etc.) the
+same specific-dependent instruction is not straightforward when recursion is
+involved. The simplest example is when A and B each are recursive functions, and
+are equivalent. The check "are A and B equivalent" needs to start by assuming
+they are equivalent, and when a self-recursive call is found in each, that call
+is still equivalent. In practice this requires comparison of `specific_id`s,
+which in SemIR are distinct.
+
+In the general case, this analysis needs to analyze the call graph for all
+functions and build strongly connected components (SCCs). The call graph could
+either be created before lowering or built while lowering. The current
+implementation does the latter, and in a post-processing phase we can conclude
+equivalence and simplify the emitted LLVM IR by deleting unnecessary parts.
+
+A non-viable option is building the call graph based on the information "what
+are all call sites of myself, where I am a specific function", because this
+information is not available until processing the function bodies of all
+specific functions. This is an optimization done so that the definition of a
+specific isn’t emitted until a use of it is found. Building that information
+would duplicate all the lowering logic, minus the LLVM IR creation.
+
+### Function fingerprints
+
+Even with limiting the comparison of specific functions to those defined from
+the same generic, a comparison algorithm would still end up with quadratic
+complexity in the number of specifics for that generic.
+
+We define two fingerprints for each specific:
+
+1. `specific_fingerprint`: Includes all specific-dependent information.
+2. `common_fingerprint`: Includes the same except for `specific_id` information,
+   as `specific_id`s can only be determined to be equivalent after building an
+   equivalence SCC.
+
+Two specific functions are equivalent if their `specific_fingerprint`s are equal
+and are not equivalent if their `common_fingerprint`s differs. If the
+`common_fingerprint`s are equal but the `specific_fingerprint`s are not, the two
+functions may still be equivalent.
+
+Ideally, the `specific_fingerprint` can be used as a unique hash to first
+coalesce all specific functions with this same fingerprint, with no additional
+checks. Then, all remaining functions may use the `common_fingerprint` as
+another unique hash to group remaining potential candidates for coalescing.
+Then, only those with this same `common_fingerprint` are processed in a
+quadratic pass walking all calls instructions and comparing if the `specific_id`
+information is equivalent. These optimizations are not currently implemented.
+
+Note that this does not
+[coalesce non-specifics](#coalescing-duplicate-non-specific-functions).
+
+### Canonical specific to use
+
+For determining the canonical specific to use, we use a
+[disjoint set](https://en.wikipedia.org/wiki/Disjoint-set_data_structure).
+
+## Algorithm details
+
+Below is a pseudocode of the existing algorithm in
+`toolchain/lower/specific_coalescer.*`.
+
+The implementation can be found in
+[specifics_coalescer.h](/toolchain/lower/specific_coalescer.h) and
+[specifics_coalescer.cpp](/toolchain/lower/specific_coalescer.cpp).
+
+At the top level, the current algorithm first generates all function
+definitions, and once this is complete, it performs the logic to coalesce
+specifics and delete the redundant LLVM function definitions.
+
+```none
+LowerToLLVM () {
+  for all non_generic_functions
+    CreateLLVMFunctionDefinition (function, no_specific_id);
+  PerformCoalescingPostProcessing ();
+}
+```
+
+The lowering starts with all non-generic functions. While lowering these, when
+calls to specifics are encountered, it also generates definitions for those
+specific functions.
+
+For each lowered specific function definition, we create the
+`SpecificFunctionFingerprint`, which includes the
+[two fingerprints](#function-fingerprints), and a list of calls to other
+specific functions.
+
+```none
+CreateLLVMFunctionDefinition (function, specific_id) {
+  For each SemIR instruction in the function:
+    Step 1: Emit LLVM IR for the instruction
+    Step 2: If the instruction is specific-dependent, hash it and add to its `common_fingerprint`
+    Step 3: If the SemIR instruction is a call to a specific,
+      a) Create a definition for this specific_id if it doesn't exist:
+        CreateLLVMFunctionDefinition (function, specific_id);
+      b) Hash the specific_id to the current function's `specific_fingerprint`
+      c) Add the non-hashed specific_id to list of calls performed
+}
+```
+
+The logic that performs the actual coalescing analyzes all specifics. For each
+pair of two specifics, it first checks if the LLVM function types match (using a
+third hash-like fingerprint: `function_type_fingerprint` for storage
+optimization), then if these are equivalent based on the
+`SpecificFunctionFingerprint`. For each pair of equivalent functions found (in a
+callgraph SCC), one function will be marked non-canonical: its uses are replaced
+with the canonical one and its definition will ultimately be deleted.
+
+```none
+PerformCoalescingPostProcessing () {
+  for each two specifics of the same generic {
+    if function_type_fingerprints differ {
+      track as non-equivalent
+      continue
+    }
+
+    add the two specifics to assumed equivalent specifics list
+    if (CheckIfEquivalent(two specifics, assumed equivalent specifics list)) {
+      for each two equivalent specifics found {
+        find the canonical specific & mark the duplicates for replacement/deletion
+    }
+  }
+  replace all duplicate specifics with the respective canonical specifics
+  and delete all replaced LLVM function definitions.
+}
+
+```
+
+The equivalence check for specifics based on the constructed
+`SpecificFunctionFingerprint` can make an early non-equivalence determination
+based on the `common_fingerprint`s, and an early equivalence determination based
+on the `specific_fingerprint`s. Otherwise, it uses the call list and recurses to
+make the determination for all functions in the SCC call graph (in practice the
+implementation uses a worklist to avoid the recursion).
+
+```none
+CheckIfEquivalent(two specifics, &assumed equivalent specifics) -> bool {
+  if common_fingerprints are non-equal {
+    track as non-equivalent specifics
+    return false
+  }
+  if specific_fingerprints are equal {
+    track as equivalent specifics
+    return true
+  }
+  if already tracked as equivalent or assumed equivalent specifics {
+    return true
+  }
+
+  for each of the calls in each of the specifics {
+    if the functions called are the same or already equivalent or assumed equivalent specifics {
+      continue
+    }
+    if the functions called are already non-equivalent specifics {
+      return false
+    }
+    add <pair of calls> to assumed equivalent specifics
+    if !CheckIfEquivalent(specifics in <pair of calls>, assumed equivalent specifics) {
+      return false;
+    }
+  }
+}
+```
+
+## Alternatives considered
+
+### Coalescing in the front-end vs back-end?
+
+An alternative considered was not doing any coalescing in the front-end and
+relying on LLVM to make the analysis and optimization. The current choice was
+made based on the expectation that such an
+[LLVM pass](https://llvm.org/docs/MergeFunctions.html) would be more costly in
+terms of compile-time. The relative cost has not yet been evaluated.
+
+### When to do coalescing in the front-end?
+
+The analysis and coalescing could be done prior to lowering, after
+specialization. The advantage of that choice would be avoiding to lower
+duplicate LLVM functions and then removing the duplicates. The disadvantage of
+that choice would be duplicating much of the lowering logic, currently necessary
+to make the equivalence determination.
+
+### Compile-time trade-offs
+
+Not doing any coalescing is also expected to increase the back-end codegen time
+more than performing the analysis and deduplication. This can be evaluated in
+practice and the feature disabled if found to be too costly.
+
+### Coalescing duplicate non-specific functions
+
+We could coalesce duplicate functions in non-specific cases, similar to lld's
+[Identical Code Folding](https://lld.llvm.org/NewLLD.html#glossary) or LLVM's
+[MergeFunctions pass](https://llvm.org/docs/MergeFunctions.html). This would
+require fingerprinting all instructions in all functions, whereas specific
+coalescing can focus on cases that only Carbon's front-end knows about. Carbon
+would also be restricted to coalescing functions in a single compilation unit,
+which would require replacing function definitions that allow external calls
+with a placeholder that calls the coalesced definition. We don't expect
+sufficient advantages over existing support.
+
+## Opportunities for further improvement
+
+The current implemented algorithm can be improved with at least the following:
+
+-   The `specific_fingerprint` can be used to already bucket specifics that can
+    be coalesced right away.
+-   The remaining ones can be pre-bucketed such that only the specifics with the
+    same `common_fingerprint` have their list of calls further compared (linear
+    in the number of specific calls inside the functions) to determine SCCs that
+    may be equivalent.
+
+This should reduce the complexity from the current O(N^2), with N=number of
+specifics for a generic, to O(M^2), with M being the number of specifics for a
+generic that have different `specific_fingerprint` and equal
+`common_fingerprint` (expectation is that M << N).
+
+An additional potential improvement is defining the function fingerprints in a
+manner that is translation-unit independent, so this can be used in the mangled
+name, and the same function name emitted. This does not currently occur, as the
+two fingerprints use internal SemIR identifiers (`function_id` and `specific_id`
+respectively).

toolchain/docs/lower.md

@@ -136,6 +136,9 @@ function should follow the same pattern, adding a getter on `FunctionContext`
 that adds the information to the fingerprint, and returns a `*InFile` wrapper
 struct if the result contains any `TypeId`s.
 
+Additional details can be found in:
+[Coalescing generic functions emitted when lowering to LLVM IR](coalesce_generic_lowering.md).
+
 ## Mangling
 
 Part of lowering is choosing deterministically unique identifiers for each