cmd/compile: decrease inlining call cost from 60 to 57

A Go user made a well-documented request for a slightly
lower threshold.  I tested against a selection of other
people's benchmarks, and saw a tiny benefit (possibly noise)
at equally tiny cost, and no unpleasant surprises observed
in benchmarking.

I.e., might help, doesn't hurt, low risk, request was
delivered on a silver platter.

It did, however, change the behavior of one test because
now bytes.Buffer.Grow is eligible for inlining.

Updates #19348.

Change-Id: I85e3088a4911290872b8c6bda9601b5354c48695
Reviewed-on: https://go-review.googlesource.com/c/151977
Run-TryBot: David Chase <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Keith Randall <[email protected]>

Author: dr2chase

src/cmd/compile/internal/gc/inl.go

@@ -39,7 +39,7 @@ const (
 	inlineMaxBudget       = 80
 	inlineExtraAppendCost = 0
 	// default is to inline if there's at most one call. -l=4 overrides this by using 1 instead.
-	inlineExtraCallCost  = inlineMaxBudget * 3 / 4
+	inlineExtraCallCost  = 57              // 57 was benchmarked to provided most benefit with no bad surprises; see https://github.com/golang/go/issues/19348#issuecomment-439370742
 	inlineExtraPanicCost = 1               // do not penalize inlining panics.
 	inlineExtraThrowCost = inlineMaxBudget // with current (2018-05/1.11) code, inlining runtime.throw does not help.
 

test/fixedbugs/issue7921.go

@@ -17,41 +17,41 @@ func bufferNotEscape() string {
 	// copied during String() call, but object "handle" itself
 	// can be stack-allocated.
 	var b bytes.Buffer
-	b.WriteString("123") // ERROR "b does not escape"
-	b.Write([]byte{'4'}) // ERROR "b does not escape" "\[\]byte literal does not escape"
-	return b.String()    // ERROR "b does not escape" "inlining call" "string\(bytes\.b\.buf\[bytes.b.off:\]\) escapes to heap"
+	b.WriteString("123") // ERROR "bufferNotEscape b does not escape$"
+	b.Write([]byte{'4'}) // ERROR "bufferNotEscape \[\]byte literal does not escape$" "bufferNotEscape b does not escape$"
+	return b.String()    // ERROR "bufferNotEscape b does not escape$" "inlining call to bytes.\(\*Buffer\).String$" "string\(bytes.b.buf\[bytes.b.off:\]\) escapes to heap$"
 }
 
-func bufferNoEscape2(xs []string) int { // ERROR "xs does not escape"
-	b := bytes.NewBuffer(make([]byte, 0, 64)) // ERROR "inlining call" "make\(\[\]byte, 0, 64\) does not escape" "&bytes.Buffer literal does not escape"
+func bufferNoEscape2(xs []string) int { // ERROR "bufferNoEscape2 xs does not escape$"
+	b := bytes.NewBuffer(make([]byte, 0, 64)) // ERROR "bufferNoEscape2 &bytes.Buffer literal does not escape$" "bufferNoEscape2 make\(\[\]byte, 0, 64\) does not escape$" "inlining call to bytes.NewBuffer$"
 	for _, x := range xs {
 		b.WriteString(x)
 	}
-	return b.Len() // ERROR "inlining call"
+	return b.Len() // ERROR "inlining call to bytes.\(\*Buffer\).Len$"
 }
 
-func bufferNoEscape3(xs []string) string { // ERROR "xs does not escape"
-	b := bytes.NewBuffer(make([]byte, 0, 64)) // ERROR "inlining call" "make\(\[\]byte, 0, 64\) does not escape" "&bytes.Buffer literal does not escape"
+func bufferNoEscape3(xs []string) string { // ERROR "bufferNoEscape3 xs does not escape$"
+	b := bytes.NewBuffer(make([]byte, 0, 64)) // ERROR "bufferNoEscape3 &bytes.Buffer literal does not escape$" "bufferNoEscape3 make\(\[\]byte, 0, 64\) does not escape$" "inlining call to bytes.NewBuffer$"
 	for _, x := range xs {
 		b.WriteString(x)
 		b.WriteByte(',')
 	}
-	return b.String() // ERROR "inlining call" "string\(bytes.b.buf\[bytes\.b\.off:\]\) escapes to heap"
+	return b.String() // ERROR "inlining call to bytes.\(\*Buffer\).String$" "string\(bytes.b.buf\[bytes.b.off:\]\) escapes to heap$"
 }
 
 func bufferNoEscape4() []byte {
 	var b bytes.Buffer
-	b.Grow(64)       // ERROR "b does not escape"
-	useBuffer(&b)    // ERROR "&b does not escape"
-	return b.Bytes() // ERROR "inlining call" "b does not escape"
+	b.Grow(64)       // ERROR "bufferNoEscape4 b does not escape$" "bufferNoEscape4 ignoring self-assignment in bytes.b.buf = bytes.b.buf\[:bytes.m·3\]$" "inlining call to bytes.\(\*Buffer\).Grow$"
+	useBuffer(&b)    // ERROR "bufferNoEscape4 &b does not escape$"
+	return b.Bytes() // ERROR "bufferNoEscape4 b does not escape$" "inlining call to bytes.\(\*Buffer\).Bytes$"
 }
 
-func bufferNoEscape5() { // ERROR "can inline bufferNoEscape5"
-	b := bytes.NewBuffer(make([]byte, 0, 128)) // ERROR "inlining call" "make\(\[\]byte, 0, 128\) does not escape" "&bytes.Buffer literal does not escape"
+func bufferNoEscape5() { // ERROR "can inline bufferNoEscape5$"
+	b := bytes.NewBuffer(make([]byte, 0, 128)) // ERROR "bufferNoEscape5 &bytes.Buffer literal does not escape$" "bufferNoEscape5 make\(\[\]byte, 0, 128\) does not escape$" "inlining call to bytes.NewBuffer$"
 	useBuffer(b)
 }
 
 //go:noinline
-func useBuffer(b *bytes.Buffer) { // ERROR "b does not escape"
+func useBuffer(b *bytes.Buffer) { // ERROR "useBuffer b does not escape$"
 	b.WriteString("1234")
 }