[dev.link] cmd/link: use function address directly in pclntab generation

If we are internal linking a static executable, in pclntab
generation, the function addresses are known, so we can just use
them directly instead of emitting relocations.

For external linking or other build modes,  we are generating a
relocatable binary so we still need to emit relocations.

Reduce some allocations: for linking cmd/compile,

name           old alloc/op   new alloc/op   delta
Pclntab_GC       38.8MB ± 0%    36.4MB ± 0%   -6.19%  (p=0.008 n=5+5)

TODO: can we also do this in DWARF generation?

Change-Id: I43920d930ab1da97c205871027e01844a07a5e60
Reviewed-on: https://go-review.googlesource.com/c/go/+/228478
Run-TryBot: Cherry Zhang <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Than McIntosh <[email protected]>

src/cmd/link/internal/ld/pcln.go

@@ -325,6 +325,21 @@ func (ctxt *Link) pclntab() loader.Bitmap {
 		return newoff
 	}
 
+	setAddr := (*loader.SymbolBuilder).SetAddrPlus
+	if ctxt.IsExe() && ctxt.IsInternal() {
+		// Internal linking static executable. At this point the function
+		// addresses are known, so we can just use them instead of emitting
+		// relocations.
+		// For other cases we are generating a relocatable binary so we
+		// still need to emit relocations.
+		setAddr = func(s *loader.SymbolBuilder, arch *sys.Arch, off int64, tgt loader.Sym, add int64) int64 {
+			if v := ldr.SymValue(tgt); v != 0 {
+				return s.SetUint(arch, off, uint64(v+add))
+			}
+			return s.SetAddrPlus(arch, off, tgt, add)
+		}
+	}
+
 	pcsp := sym.Pcdata{}
 	pcfile := sym.Pcdata{}
 	pcline := sym.Pcdata{}
@@ -347,7 +362,7 @@ func (ctxt *Link) pclntab() loader.Bitmap {
 			// invalid funcoff value to mark the hole. See also
 			// runtime/symtab.go:findfunc
 			prevFuncSize := int64(ldr.SymSize(prevFunc))
-			ftab.SetAddrPlus(ctxt.Arch, 8+int64(ctxt.Arch.PtrSize)+int64(nfunc)*2*int64(ctxt.Arch.PtrSize), prevFunc, prevFuncSize)
+			setAddr(ftab, ctxt.Arch, 8+int64(ctxt.Arch.PtrSize)+int64(nfunc)*2*int64(ctxt.Arch.PtrSize), prevFunc, prevFuncSize)
 			ftab.SetUint(ctxt.Arch, 8+int64(ctxt.Arch.PtrSize)+int64(nfunc)*2*int64(ctxt.Arch.PtrSize)+int64(ctxt.Arch.PtrSize), ^uint64(0))
 			nfunc++
 		}
@@ -397,10 +412,7 @@ func (ctxt *Link) pclntab() loader.Bitmap {
 		funcstart := int32(dSize)
 		funcstart += int32(-dSize) & (int32(ctxt.Arch.PtrSize) - 1) // align to ptrsize
 
-		// NB: for the static binary internal-link case, we could just
-		// emit the symbol value instead of creating a relocation here
-		// (might speed things up for that case).
-		ftab.SetAddrPlus(ctxt.Arch, 8+int64(ctxt.Arch.PtrSize)+int64(nfunc)*2*int64(ctxt.Arch.PtrSize), s, 0)
+		setAddr(ftab, ctxt.Arch, 8+int64(ctxt.Arch.PtrSize)+int64(nfunc)*2*int64(ctxt.Arch.PtrSize), s, 0)
 		ftab.SetUint(ctxt.Arch, 8+int64(ctxt.Arch.PtrSize)+int64(nfunc)*2*int64(ctxt.Arch.PtrSize)+int64(ctxt.Arch.PtrSize), uint64(funcstart))
 
 		// Write runtime._func. Keep in sync with ../../../../runtime/runtime2.go:/_func
@@ -416,7 +428,7 @@ func (ctxt *Link) pclntab() loader.Bitmap {
 		ftab.Grow(int64(end))
 
 		// entry uintptr
-		off = int32(ftab.SetAddrPlus(ctxt.Arch, int64(off), s, 0))
+		off = int32(setAddr(ftab, ctxt.Arch, int64(off), s, 0))
 
 		// name int32
 		sn := ldr.SymName(s)
@@ -497,7 +509,7 @@ func (ctxt *Link) pclntab() loader.Bitmap {
 				}
 				// TODO: Dedup.
 				funcdataBytes += int64(len(ldr.Data(funcdata[i])))
-				ftab.SetAddrPlus(ctxt.Arch, dataoff, funcdata[i], funcdataoff[i])
+				setAddr(ftab, ctxt.Arch, dataoff, funcdata[i], funcdataoff[i])
 			}
 			off += int32(len(funcdata)) * int32(ctxt.Arch.PtrSize)
 		}
@@ -513,7 +525,7 @@ func (ctxt *Link) pclntab() loader.Bitmap {
 	last := ctxt.Textp2[len(ctxt.Textp2)-1]
 	pclntabLastFunc2 = last
 	// Final entry of table is just end pc.
-	ftab.SetAddrPlus(ctxt.Arch, 8+int64(ctxt.Arch.PtrSize)+int64(nfunc)*2*int64(ctxt.Arch.PtrSize), last, ldr.SymSize(last))
+	setAddr(ftab, ctxt.Arch, 8+int64(ctxt.Arch.PtrSize)+int64(nfunc)*2*int64(ctxt.Arch.PtrSize), last, ldr.SymSize(last))
 
 	// Start file table.
 	dSize := len(ftab.Data())

src/cmd/link/internal/ld/target.go

@@ -28,6 +28,10 @@ type Target struct {
 // Target type functions
 //
 
+func (t *Target) IsExe() bool {
+	return t.BuildMode == BuildModeExe
+}
+
 func (t *Target) IsShared() bool {
 	return t.BuildMode == BuildModeShared
 }