cmd/compile,runtime: pass only ptr and len to some runtime calls

Some runtime calls accept a slice, but only use ptr and len.
This change modifies most such routines to accept only ptr and len.

After this change, the only runtime calls that accept an unnecessary
cap arg are concatstrings and slicerunetostring.
Neither is particularly common, and both are complicated to modify.

Negligible compiler performance impact. Shrinks binaries a little.
There are only a few regressions; the one I investigated was
due to register allocation fluctuation.

Passes 'go test -race std cmd', modulo #38265 and #38266.
Wow, does that take a long time to run.

Updates #36890

file      before    after     Δ       %       
compile   19655024  19655152  +128    +0.001% 
cover     5244840   5236648   -8192   -0.156% 
dist      3662376   3658280   -4096   -0.112% 
link      6680056   6675960   -4096   -0.061% 
pprof     14789844  14777556  -12288  -0.083% 
test2json 2824744   2820648   -4096   -0.145% 
trace     11647876  11639684  -8192   -0.070% 
vet       8260472   8256376   -4096   -0.050% 
total     115163736 115118808 -44928  -0.039% 

Change-Id: Idb29fa6a81d6a82bfd3b65740b98cf3275ca0a78
Reviewed-on: https://go-review.googlesource.com/c/go/+/227163
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

1 files changed, 26 insertions, 21 deletions

diff --git a/src/runtime/string.go b/src/runtime/string.go
index 7dc0bd789f..0515b56573 100644
--- a/src/runtime/string.go
+++ b/src/runtime/string.go
@@ -71,27 +71,30 @@ func concatstring5(buf *tmpBuf, a [5]string) string {
 	return concatstrings(buf, a[:])
 }
 
+// slicebytetostring converts a byte slice to a string.
+// It is inserted by the compiler into generated code.
+// ptr is a pointer to the first element of the slice;
+// n is the length of the slice.
 // Buf is a fixed-size buffer for the result,
 // it is not nil if the result does not escape.
-func slicebytetostring(buf *tmpBuf, b []byte) (str string) {
-	l := len(b)
-	if l == 0 {
+func slicebytetostring(buf *tmpBuf, ptr *byte, n int) (str string) {
+	if n == 0 {
 		// Turns out to be a relatively common case.
 		// Consider that you want to parse out data between parens in "foo()bar",
 		// you find the indices and convert the subslice to string.
 		return ""
 	}
 	if raceenabled {
-		racereadrangepc(unsafe.Pointer(&b[0]),
-			uintptr(l),
+		racereadrangepc(unsafe.Pointer(ptr),
+			uintptr(n),
 			getcallerpc(),
 			funcPC(slicebytetostring))
 	}
 	if msanenabled {
-		msanread(unsafe.Pointer(&b[0]), uintptr(l))
+		msanread(unsafe.Pointer(ptr), uintptr(n))
 	}
-	if l == 1 {
-		p := unsafe.Pointer(&staticuint64s[b[0]])
+	if n == 1 {
+		p := unsafe.Pointer(&staticuint64s[*ptr])
 		if sys.BigEndian {
 			p = add(p, 7)
 		}
@@ -101,14 +104,14 @@ func slicebytetostring(buf *tmpBuf, b []byte) (str string) {
 	}
 
 	var p unsafe.Pointer
-	if buf != nil && len(b) <= len(buf) {
+	if buf != nil && n <= len(buf) {
 		p = unsafe.Pointer(buf)
 	} else {
-		p = mallocgc(uintptr(len(b)), nil, false)
+		p = mallocgc(uintptr(n), nil, false)
 	}
 	stringStructOf(&str).str = p
-	stringStructOf(&str).len = len(b)
-	memmove(p, (*(*slice)(unsafe.Pointer(&b))).array, uintptr(len(b)))
+	stringStructOf(&str).len = n
+	memmove(p, unsafe.Pointer(ptr), uintptr(n))
 	return
 }
 
@@ -123,7 +126,7 @@ func stringDataOnStack(s string) bool {
 func rawstringtmp(buf *tmpBuf, l int) (s string, b []byte) {
 	if buf != nil && l <= len(buf) {
 		b = buf[:l]
-		s = slicebytetostringtmp(b)
+		s = slicebytetostringtmp(&b[0], len(b))
 	} else {
 		s, b = rawstring(l)
 	}
@@ -144,17 +147,19 @@ func rawstringtmp(buf *tmpBuf, l int) (s string, b []byte) {
 //   where k is []byte, T1 to Tn is a nesting of struct and array literals.
 // - Used for "<"+string(b)+">" concatenation where b is []byte.
 // - Used for string(b)=="foo" comparison where b is []byte.
-func slicebytetostringtmp(b []byte) string {
-	if raceenabled && len(b) > 0 {
-		racereadrangepc(unsafe.Pointer(&b[0]),
-			uintptr(len(b)),
+func slicebytetostringtmp(ptr *byte, n int) (str string) {
+	if raceenabled && n > 0 {
+		racereadrangepc(unsafe.Pointer(ptr),
+			uintptr(n),
 			getcallerpc(),
 			funcPC(slicebytetostringtmp))
 	}
-	if msanenabled && len(b) > 0 {
-		msanread(unsafe.Pointer(&b[0]), uintptr(len(b)))
+	if msanenabled && n > 0 {
+		msanread(unsafe.Pointer(ptr), uintptr(n))
 	}
-	return *(*string)(unsafe.Pointer(&b))
+	stringStructOf(&str).str = unsafe.Pointer(ptr)
+	stringStructOf(&str).len = n
+	return
 }
 
 func stringtoslicebyte(buf *tmpBuf, s string) []byte {
@@ -239,7 +244,7 @@ func intstring(buf *[4]byte, v int64) (s string) {
 	var b []byte
 	if buf != nil {
 		b = buf[:]
-		s = slicebytetostringtmp(b)
+		s = slicebytetostringtmp(&b[0], len(b))
 	} else {
 		s, b = rawstring(4)
 	}