cmd/link,runtime: remove functab relocations

Use an offset from runtime.text instead.
This removes the last relocation from functab generation,
which lets us simplify that code.

size      before    after     Δ        %
addr2line 3680818   3652498   -28320   -0.769%
api       4944850   4892418   -52432   -1.060%
asm       4757586   4711266   -46320   -0.974%
buildid   2418546   2392578   -25968   -1.074%
cgo       4197346   4164818   -32528   -0.775%
compile   22076882  21875890  -200992  -0.910%
cover     4411362   4358418   -52944   -1.200%
dist      3091346   3062738   -28608   -0.925%
doc       3563234   3532610   -30624   -0.859%
fix       3020658   2991666   -28992   -0.960%
link      6164642   6110834   -53808   -0.873%
nm        3646818   3618482   -28336   -0.777%
objdump   4012594   3983042   -29552   -0.736%
pack      2153554   2128338   -25216   -1.171%
pprof     13011666  12870114  -141552  -1.088%
test2json 2383906   2357554   -26352   -1.105%
trace     9736514   9631186   -105328  -1.082%
vet       6655058   6580370   -74688   -1.122%
total     103927380 102914820 -1012560 -0.974%

relocs    before  after   Δ       %
addr2line 25069   22709   -2360   -9.414%
api       17176   13321   -3855   -22.444%
asm       18271   15630   -2641   -14.455%
buildid   9233    7352    -1881   -20.373%
cgo       16222   13044   -3178   -19.591%
compile   60421   46299   -14122  -23.373%
cover     18479   14526   -3953   -21.392%
dist      10135   7733    -2402   -23.700%
doc       12735   9940    -2795   -21.947%
fix       10820   8341    -2479   -22.911%
link      21849   17785   -4064   -18.600%
nm        24988   22642   -2346   -9.389%
objdump   26060   23462   -2598   -9.969%
pack      7665    5936    -1729   -22.557%
pprof     60764   50998   -9766   -16.072%
test2json 8389    6431    -1958   -23.340%
trace     37180   29382   -7798   -20.974%
vet       24044   19055   -4989   -20.749%
total     409499  334585  -74914  -18.294%


Caching the field size in debug/gosym.funcTab
avoids a 20% PCToLine performance regression.

name            old time/op    new time/op    delta
115/LineToPC-8    56.4µs ± 3%    57.3µs ± 2%  +1.66%  (p=0.006 n=15+13)
115/PCToLine-8     188ns ± 2%     190ns ± 3%  +1.46%  (p=0.030 n=15+15)


Change-Id: I2816a1b28e62b01852e3b306f08546f1e56cd5ac
Reviewed-on: https://go-review.googlesource.com/c/go/+/352191
Trust: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>

1 files changed, 12 insertions, 4 deletions

diff --git a/src/debug/gosym/pclntab.go b/src/debug/gosym/pclntab.go
index 8fe45decd6..134cb3d194 100644
--- a/src/debug/gosym/pclntab.go
+++ b/src/debug/gosym/pclntab.go
@@ -373,18 +373,22 @@ func (t *LineTable) string(off uint32) string {
 
 // functabFieldSize returns the size in bytes of a single functab field.
 func (t *LineTable) functabFieldSize() int {
+	if t.version >= ver118 {
+		return 4
+	}
 	return int(t.ptrsize)
 }
 
 // funcTab returns t's funcTab.
 func (t *LineTable) funcTab() funcTab {
-	return funcTab{t}
+	return funcTab{LineTable: t, sz: t.functabFieldSize()}
 }
 
 // funcTab is memory corresponding to a slice of functab structs, followed by an invalid PC.
 // A functab struct is a PC and a func offset.
 type funcTab struct {
 	*LineTable
+	sz int // cached result of t.functabFieldSize
 }
 
 // Count returns the number of func entries in f.
@@ -394,17 +398,21 @@ func (f funcTab) Count() int {
 
 // pc returns the PC of the i'th func in f.
 func (f funcTab) pc(i int) uint64 {
-	return f.uint(f.functab[2*i*f.functabFieldSize():])
+	u := f.uint(f.functab[2*i*f.sz:])
+	if f.version >= ver118 {
+		u += uint64(f.textStart)
+	}
+	return u
 }
 
 // funcOff returns the funcdata offset of the i'th func in f.
 func (f funcTab) funcOff(i int) uint64 {
-	return f.uint(f.functab[(2*i+1)*f.functabFieldSize():])
+	return f.uint(f.functab[(2*i+1)*f.sz:])
 }
 
 // uint returns the uint stored at b.
 func (f funcTab) uint(b []byte) uint64 {
-	if f.functabFieldSize() == 4 {
+	if f.sz == 4 {
 		return uint64(f.binary.Uint32(b))
 	}
 	return f.binary.Uint64(b)