cmd/compile: allow linker to drop string headers when not needed

Compiling a simple file containing a slice of 100,000 strings,
the size of the resulting binary dropped from 5,896,224 bytes
to 3,495,968 bytes, which is the expected 2,400,000 bytes,
give or take.

Fixes #7384.

Change-Id: I3e551b5a1395b523a41b33518d81a1bf28da0906
Reviewed-on: https://go-review.googlesource.com/11698
Reviewed-by: Austin Clements <austin@google.com>

diff --git a/src/cmd/compile/internal/gc/obj.go b/src/cmd/compile/internal/gc/obj.go
index 22b10d908301443ab6f406c659bc8c000a056add..d2ac813a88b65916d9748ea8d94c3d05bab6fd06 100644 (file)
--- a/src/cmd/compile/internal/gc/obj.go
+++ b/src/cmd/compile/internal/gc/obj.go
@@ -200,7 +200,7 @@ func duintptr(s *Sym, off int, v uint64) int {
 
 var stringsym_gen int
 
-func stringsym(s string) *Sym {
+func stringsym(s string) (hdr, data *Sym) {
        var symname string
        var pkg *Pkg
        if len(s) > 100 {
@@ -217,36 +217,44 @@ func stringsym(s string) *Sym {
                pkg = gostringpkg
        }
 
-       sym := Pkglookup(symname, pkg)
+       symhdr := Pkglookup("hdr."+symname, pkg)
+       symdata := Pkglookup(symname, pkg)
 
        // SymUniq flag indicates that data is generated already
-       if sym.Flags&SymUniq != 0 {
-               return sym
+       if symhdr.Flags&SymUniq != 0 {
+               return symhdr, symdata
        }
-       sym.Flags |= SymUniq
-       sym.Def = newname(sym)
-
-       off := 0
+       symhdr.Flags |= SymUniq
+       symhdr.Def = newname(symhdr)
 
        // string header
-       off = dsymptr(sym, off, sym, Widthptr+Widthint)
-       off = duintxx(sym, off, uint64(len(s)), Widthint)
+       off := 0
+       off = dsymptr(symhdr, off, symdata, 0)
+       off = duintxx(symhdr, off, uint64(len(s)), Widthint)
+       ggloblsym(symhdr, int32(off), obj.DUPOK|obj.RODATA|obj.LOCAL)
 
        // string data
+       if symdata.Flags&SymUniq != 0 {
+               return symhdr, symdata
+       }
+       symdata.Flags |= SymUniq
+       symdata.Def = newname(symdata)
+
+       off = 0
        var m int
        for n := 0; n < len(s); n += m {
                m = 8
                if m > len(s)-n {
                        m = len(s) - n
                }
-               off = dsname(sym, off, s[n:n+m])
+               off = dsname(symdata, off, s[n:n+m])
        }
 
-       off = duint8(sym, off, 0)                    // terminating NUL for runtime
+       off = duint8(symdata, off, 0)                // terminating NUL for runtime
        off = (off + Widthptr - 1) &^ (Widthptr - 1) // round to pointer alignment
-       ggloblsym(sym, int32(off), obj.DUPOK|obj.RODATA|obj.LOCAL)
+       ggloblsym(symdata, int32(off), obj.DUPOK|obj.RODATA|obj.LOCAL)
 
-       return sym
+       return symhdr, symdata
 }
 
 var slicebytes_gen int
@@ -299,22 +307,22 @@ func dstringptr(s *Sym, off int, str string) int {
 }
 
 func Datastring(s string, a *obj.Addr) {
-       sym := stringsym(s)
+       _, symdata := stringsym(s)
        a.Type = obj.TYPE_MEM
        a.Name = obj.NAME_EXTERN
-       a.Sym = Linksym(sym)
-       a.Node = sym.Def
-       a.Offset = int64(Widthptr) + int64(Widthint) // skip header
+       a.Sym = Linksym(symdata)
+       a.Node = symdata.Def
+       a.Offset = 0
        a.Etype = Simtype[TINT]
 }
 
 func datagostring(sval string, a *obj.Addr) {
-       sym := stringsym(sval)
+       symhdr, _ := stringsym(sval)
        a.Type = obj.TYPE_MEM
        a.Name = obj.NAME_EXTERN
-       a.Sym = Linksym(sym)
-       a.Node = sym.Def
-       a.Offset = 0 // header
+       a.Sym = Linksym(symhdr)
+       a.Node = symhdr.Def
+       a.Offset = 0
        a.Etype = TSTRING
 }