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all: add support for ThinLTO

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ThinLTO optimizes across LLVM modules at link time. This means that
optimizations (such as inlining and const-propagation) are possible
between C and Go. This makes this change especially useful for CGo, but
not just for CGo. By doing some optimizations at link time, the linker
can discard some unused functions and this leads to a size reduction on
average. It does increase code size in some cases, but that's true for
most optimizations.

I've excluded a number of targets for now (wasm, avr, xtensa, windows,
macos). They can probably be supported with some more work, but that
should be done in separate PRs.

Overall, this change results in an average 3.24% size reduction over all
the tinygo.org/x/drivers smoke tests.

TODO: this commit runs part of the pass pipeline twice. We should set
the PrepareForThinLTO flag in the PassManagerBuilder for even further
reduced code size (0.7%) and improved compilation speed.
This commit is contained in:
Ayke van Laethem
2022-02-07 15:19:25 +01:00
committed by Ron Evans
parent d4b1467e4c
commit 603fff78d4
8 changed files with 121 additions and 25 deletions
Download Patch File Download Diff File Expand all files Collapse all files
builder/build.go
+49 -15
View File
@@ -105,6 +105,15 @@ func Build(pkgName, outpath string, config *compileopts.Config, action func(Buil
defer os.RemoveAll(dir)
}
// Look up the build cache directory, which is used to speed up incremental
// builds.
cacheDir := goenv.Get("GOCACHE")
if cacheDir == "off" {
// Use temporary build directory instead, effectively disabling the
// build cache.
cacheDir = dir
}
// Check for a libc dependency.
// As a side effect, this also creates the headers for the given libc, if
// the libc needs them.
@@ -238,12 +247,6 @@ func Build(pkgName, outpath string, config *compileopts.Config, action func(Buil
// Determine the path of the bitcode file (which is a serialized version
// of a LLVM module).
cacheDir := goenv.Get("GOCACHE")
if cacheDir == "off" {
// Use temporary build directory instead, effectively disabling the
// build cache.
cacheDir = dir
}
bitcodePath := filepath.Join(cacheDir, "pkg-"+hex.EncodeToString(hash[:])+".bc")
packageBitcodePaths[pkg.ImportPath] = bitcodePath
@@ -416,7 +419,7 @@ func Build(pkgName, outpath string, config *compileopts.Config, action func(Buil
// Load and link all the bitcode files. This does not yet optimize
// anything, it only links the bitcode files together.
ctx := llvm.NewContext()
mod = ctx.NewModule("")
mod = ctx.NewModule("main")
for _, pkg := range lprogram.Sorted() {
pkgMod, err := ctx.ParseBitcodeFile(packageBitcodePaths[pkg.ImportPath])
if err != nil {
@@ -512,8 +515,14 @@ func Build(pkgName, outpath string, config *compileopts.Config, action func(Buil
}
return ioutil.WriteFile(outpath, llvmBuf.Bytes(), 0666)
case ".bc":
data := llvm.WriteBitcodeToMemoryBuffer(mod).Bytes()
return ioutil.WriteFile(outpath, data, 0666)
var buf llvm.MemoryBuffer
if config.UseThinLTO() {
buf = llvm.WriteThinLTOBitcodeToMemoryBuffer(mod)
} else {
buf = llvm.WriteBitcodeToMemoryBuffer(mod)
}
defer buf.Dispose()
return ioutil.WriteFile(outpath, buf.Bytes(), 0666)
case ".ll":
data := []byte(mod.String())
return ioutil.WriteFile(outpath, data, 0666)
@@ -533,10 +542,17 @@ func Build(pkgName, outpath string, config *compileopts.Config, action func(Buil
dependencies: []*compileJob{programJob},
result: objfile,
run: func(*compileJob) error {
llvmBuf, err := machine.EmitToMemoryBuffer(mod, llvm.ObjectFile)
if err != nil {
return err
var llvmBuf llvm.MemoryBuffer
if config.UseThinLTO() {
llvmBuf = llvm.WriteThinLTOBitcodeToMemoryBuffer(mod)
} else {
var err error
llvmBuf, err = machine.EmitToMemoryBuffer(mod, llvm.ObjectFile)
if err != nil {
return err
}
}
defer llvmBuf.Dispose()
return ioutil.WriteFile(objfile, llvmBuf.Bytes(), 0666)
},
}
@@ -569,7 +585,7 @@ func Build(pkgName, outpath string, config *compileopts.Config, action func(Buil
job := &compileJob{
description: "compile extra file " + path,
run: func(job *compileJob) error {
result, err := compileAndCacheCFile(abspath, dir, config.CFlags(), config.Options.PrintCommands)
result, err := compileAndCacheCFile(abspath, dir, config.CFlags(), config.UseThinLTO(), config.Options.PrintCommands)
job.result = result
return err
},
@@ -587,7 +603,7 @@ func Build(pkgName, outpath string, config *compileopts.Config, action func(Buil
job := &compileJob{
description: "compile CGo file " + abspath,
run: func(job *compileJob) error {
result, err := compileAndCacheCFile(abspath, dir, pkg.CFlags, config.Options.PrintCommands)
result, err := compileAndCacheCFile(abspath, dir, pkg.CFlags, config.UseThinLTO(), config.Options.PrintCommands)
job.result = result
return err
},
@@ -656,6 +672,24 @@ func Build(pkgName, outpath string, config *compileopts.Config, action func(Buil
if config.Options.PrintCommands != nil {
config.Options.PrintCommands(config.Target.Linker, ldflags...)
}
if config.UseThinLTO() {
ldflags = append(ldflags,
"--thinlto-cache-dir="+filepath.Join(cacheDir, "thinlto"),
"-plugin-opt=mcpu="+config.CPU(),
"-plugin-opt=O"+strconv.Itoa(optLevel),
"-plugin-opt=thinlto")
if config.CodeModel() != "default" {
ldflags = append(ldflags,
"-mllvm", "-code-model="+config.CodeModel())
}
if sizeLevel >= 2 {
// Workaround with roughly the same effect as
// https://reviews.llvm.org/D119342.
// Can hopefully be removed in LLVM 15.
ldflags = append(ldflags,
"-mllvm", "--rotation-max-header-size=0")
}
}
err = link(config.Target.Linker, ldflags...)
if err != nil {
return &commandError{"failed to link", executable, err}
@@ -846,7 +880,7 @@ func optimizeProgram(mod llvm.Module, config *compileopts.Config) error {
}
}
if config.GOOS() != "darwin" {
if config.GOOS() != "darwin" && !config.UseThinLTO() {
transform.ApplyFunctionSections(mod) // -ffunction-sections
}