compiler: mark the source argument to the append builtin as not escaping

[eliasnaur]

No description provided.

[eliasnaur]

append may return the slice; does that make the "nocapture" attribute inappropriate? Edit: stdlib test failures indicate so.

[dgryski]

This looks cool. I wonder how many times this optimization actually triggers?

[eliasnaur]

Good question. This change is not enough for the strconv.Append* functions to allow stack allocating the destination argument. For that, I probably need an argument-escapes-to-return-value attribute/pass like big Go does. Do you have any ideas how to approach that in TinyGo?

[dgryski]

I think @aykevl had some ideas on being able to rely more on llvm for escape analysis but I don't know the status of that work.

[dgryski]

I can't seem to get this to trigger.

~/go/src/github.com/dgryski/bug/heapappend $ tinygo run -print-allocs=main main.go
/Users/dgryski/go/src/github.com/dgryski/bug/heapappend/main.go:7:13: object allocated on the heap: escapes at line 9
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~/go/src/github.com/dgryski/bug/heapappend $ cat main.go
package main

type nyb byte

//go:noinline
func (n nyb) String() string {
	buf := make([]byte, 0, 8)
	const digs = "01"
	buf = append(buf, digs[n&1])
	n >>= 1
	buf = append(buf, digs[n&1])
	n >>= 1
	buf = append(buf, digs[n&1])
	n >>= 1
	buf = append(buf, digs[n&1])
	return string(buf)
}

func main() {
	for i := 0; i < 16; i++ {
		println(nyb(i).String())
	}
}

[eliasnaur]

I taught the escape pass about LLVM's extractvalue instruction, and your program now triggers the optimization.

[dgryski]

I added some logging and ran this across the test corpus. This triggered way less frequently than I was expecting.

Logging patch:

diff --git a/transform/allocs.go b/transform/allocs.go
index da4dbbb9..d0df64a2 100644
--- a/transform/allocs.go
+++ b/transform/allocs.go
@@ -8,6 +8,7 @@ package transform
 import (
        "fmt"
        "go/token"
+       "log"
        "regexp"

        "tinygo.org/x/go-llvm"
@@ -162,6 +163,11 @@ func valueEscapesAt(value llvm.Value) llvm.Value {
                        if fn := use.CalledValue(); !fn.IsAFunction().IsNil() && fn.Name() == "runtime.sliceAppend" {
                                if at := elemEscapesAt(use, 0); !at.IsNil() {
                                        return at
+                               } else {
+                                       valPos := getPosition(value)
+                                       atPos := getPosition(at)
+                                       msg := fmt.Sprintf("sliceAppend heap->stack: %v:%v -> doesn't escape triggered %v:%v", valPos.Filename, valPos.Line, atPos.Filename, atPos.Line)
+                                       log.Println(msg)
                                }
                                break
                        }

Outputs:

cloudflare/bn256/bn256_test.go:55
VividCortex/gohistogram/weightedhistogram.go:171
VividCortex/gohistogram/numerichistogram.go:141

And from make tinygo-test-fast:
2024/11/01 21:22:18 sliceAppend heap->stack: /Users/dgryski/go/src/go.googlesource.com/go/src/encoding/base32/base32_test.go:805 -> doesn't escape triggered :0
2024/11/01 21:22:18 sliceAppend heap->stack: /Users/dgryski/go/src/go.googlesource.com/go/src/encoding/base32/base32_test.go:783 -> doesn't escape triggered :0
2024/11/01 21:22:18 sliceAppend heap->stack: /Users/dgryski/go/src/go.googlesource.com/go/src/encoding/base32/base32_test.go:766 -> doesn't escape triggered :0
2024/11/01 21:22:18 sliceAppend heap->stack: /Users/dgryski/go/src/go.googlesource.com/go/src/encoding/base32/base32_test.go:739 -> doesn't escape triggered :0
2024/11/01 21:22:19 sliceAppend heap->stack: /Users/dgryski/go/src/go.googlesource.com/go/src/encoding/base64/base64_test.go:337 -> doesn't escape triggered :0
2024/11/01 21:22:19 sliceAppend heap->stack: /Users/dgryski/go/src/go.googlesource.com/go/src/encoding/base64/base64_test.go:321 -> doesn't escape triggered :0
2024/11/01 21:22:19 sliceAppend heap->stack: /Users/dgryski/go/src/go.googlesource.com/go/src/encoding/base64/base64_test.go:305 -> doesn't escape triggered :0
2024/11/01 21:22:19 sliceAppend heap->stack: /Users/dgryski/go/src/go.googlesource.com/go/src/encoding/base64/base64_test.go:286 -> doesn't escape triggered :0

2024/11/01 21:25:15 sliceAppend heap->stack: /Users/dgryski/go/src/go.googlesource.com/go/src/strconv/atoi_test.go:657 -> doesn't escape triggered :0

[eliasnaur]

Indeed, without inter-procedural analysis this optimization only works within functions.

I'm tempted to look into porting big Go's escape analysis pass (https://github.com/golang/go/tree/master/src/cmd/compile/internal/escape), most importantly to avoid surprises where TinyGo heap allocates while big Go doesn't. Do you see any fundamental problems in that approach?

[eliasnaur]

Filed in #4579. Closing this for now.