runtime: do not zero terminate strings

On top of "tiny allocator" (cl/38750047), reduces number of allocs by 1% on json.
No code must rely on zero termination. So will also make debugging simpler,
by uncovering issues earlier.

json-1
allocated                 7949686      7915766      -0.43%
allocs                      93778        92790      -1.05%
time                    100957795     97250949      -3.67%
rest of the metrics are too noisy.

LGTM=r
R=golang-codereviews, r, bradfitz, iant
CC=golang-codereviews
https://golang.org/cl/40370061

diff --git a/src/pkg/runtime/string.goc b/src/pkg/runtime/string.goc
index 57b3546c3f3e76a7b8694dbcbe9bbcb5e1c04fa8..8eff05a8438bedf10ce12c480465b976bda128e2 100644 (file)
--- a/src/pkg/runtime/string.goc
+++ b/src/pkg/runtime/string.goc
@@ -46,10 +46,8 @@ gostringsize(intgo l)
 
        if(l == 0)
                return runtime·emptystring;
-       // leave room for NUL for C runtime (e.g., callers of getenv)
-       s.str = runtime·mallocgc(l+1, 0, FlagNoScan|FlagNoZero);
+       s.str = runtime·mallocgc(l, 0, FlagNoScan|FlagNoZero);
        s.len = l;
-       s.str[l] = 0;
        for(;;) {
                ms = runtime·maxstring;
                if((uintptr)l <= ms || runtime·casp((void**)&runtime·maxstring, (void*)ms, (void*)l))