commit 6146cf4b989f69def35dec62723d50ac2b385e8c
author Chris Wailes <chriswailes@google.com> Tue May 30 12:29:45 2023 -0700
committer Chris Wailes <chriswailes@google.com> Fri Dec 01 10:34:01 2023 -0800
tree 9f1aed189b85bbb891523b02eb8187650c8e32f6
parent 7d7d94ffd24fe4b1e8885426c92fcb2f4d034e0c

Make building Rust targets faster in eng builds

This CL makes the following changes when building an eng variant to
speed up compilation, possibly at the expense of the performance of the
generated binary:
* Disables LTO
* Enables incremental compilation
* Sets codegen units to 256 to increase parallelism (default set when
  enabling incremental compilation)

These changes reduce the time taken to compile all of Android's Rust
code from 16m20s to 12m10s.

Test: lunch aosp_oriole-eng && m clean && m rust
Bug: https://b.corp.google.com/issues/289094772
Change-Id: I97f1675c67ded69120b0c0e0fb5608aa9314a866

rust/builder.go

diff --git a/rust/builder.go b/rust/builder.go
index c855cfb..65995ed 100644
--- a/rust/builder.go
+++ b/rust/builder.go
@@ -122,8 +122,6 @@
 
 func TransformSrcToBinary(ctx ModuleContext, mainSrc android.Path, deps PathDeps, flags Flags,
 	outputFile android.WritablePath) buildOutput {
-	flags.GlobalRustFlags = append(flags.GlobalRustFlags, "-C lto=thin")
-
 	return transformSrctoCrate(ctx, mainSrc, deps, flags, outputFile, "bin")
 }
 
@@ -134,20 +132,16 @@
 
 func TransformSrctoDylib(ctx ModuleContext, mainSrc android.Path, deps PathDeps, flags Flags,
 	outputFile android.WritablePath) buildOutput {
-	flags.GlobalRustFlags = append(flags.GlobalRustFlags, "-C lto=thin")
-
 	return transformSrctoCrate(ctx, mainSrc, deps, flags, outputFile, "dylib")
 }
 
 func TransformSrctoStatic(ctx ModuleContext, mainSrc android.Path, deps PathDeps, flags Flags,
 	outputFile android.WritablePath) buildOutput {
-	flags.GlobalRustFlags = append(flags.GlobalRustFlags, "-C lto=thin")
 	return transformSrctoCrate(ctx, mainSrc, deps, flags, outputFile, "staticlib")
 }
 
 func TransformSrctoShared(ctx ModuleContext, mainSrc android.Path, deps PathDeps, flags Flags,
 	outputFile android.WritablePath) buildOutput {
-	flags.GlobalRustFlags = append(flags.GlobalRustFlags, "-C lto=thin")
 	return transformSrctoCrate(ctx, mainSrc, deps, flags, outputFile, "cdylib")
 }
 
@@ -263,6 +257,21 @@
 
 	inputs = append(inputs, main)
 
+	if ctx.Config().Eng() {
+		// Per https://doc.rust-lang.org/rustc/codegen-options/index.html#codegen-units
+		// incremental building implies codegen-units=256
+		incrementalPath := android.PathForModuleOut(ctx, "rustc-incremental").String()
+		flags.GlobalRustFlags = append(flags.GlobalRustFlags, "-C incremental="+incrementalPath)
+
+	} else {
+		flags.GlobalRustFlags = append(flags.GlobalRustFlags, "-C codegen-units=1")
+
+		if !(ctx.RustModule().Rlib() || ctx.RustModule().ProcMacro()) {
+			flags.GlobalRustFlags = append(flags.GlobalRustFlags, "-Z dylib-lto")
+			flags.GlobalRustFlags = append(flags.GlobalRustFlags, "-C lto=thin")
+		}
+	}
+
 	// Collect rustc flags
 	rustcFlags = append(rustcFlags, flags.GlobalRustFlags...)
 	rustcFlags = append(rustcFlags, flags.RustFlags...)
@@ -278,15 +287,6 @@
 	// Suppress an implicit sysroot
 	rustcFlags = append(rustcFlags, "--sysroot=/dev/null")
 
-	// Enable incremental compilation if requested by user
-	if ctx.Config().IsEnvTrue("SOONG_RUSTC_INCREMENTAL") {
-		incrementalPath := android.PathForOutput(ctx, "rustc").String()
-
-		rustcFlags = append(rustcFlags, "-C incremental="+incrementalPath)
-	} else {
-		rustcFlags = append(rustcFlags, "-C codegen-units=1")
-	}
-
 	// Disallow experimental features
 	modulePath := ctx.ModuleDir()
 	if !(android.IsThirdPartyPath(modulePath) || strings.HasPrefix(modulePath, "prebuilts")) {