From: Josh Bleecher Snyder <josharian@gmail.com>
Date: Tue, 5 Oct 2021 17:21:54 +0000 (-0700)
Subject: runtime: remove a branch from funcdata
X-Git-Tag: go1.18beta1~1042
X-Git-Url: http://www.git.cypherpunks.su/?a=commitdiff_plain;h=113b52979f48331b611e0fe7dadff97d6393ca27;p=gostls13.git

runtime: remove a branch from funcdata

name                   old time/op  new time/op  delta
StackCopyWithStkobj-8  12.1ms ± 7%  11.6ms ± 8%  -3.88%  (p=0.002 n=19+19)

Change-Id: Idf810017d541eba70bcf9c736267de9efae916d5
Reviewed-on: https://go-review.googlesource.com/c/go/+/354072
Trust: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
---

diff --git a/src/runtime/symtab.go b/src/runtime/symtab.go
index cbfe604f1b..f423957f88 100644
--- a/src/runtime/symtab.go
+++ b/src/runtime/symtab.go
@@ -1090,10 +1090,15 @@ func funcdata(f funcInfo, i uint8) unsafe.Pointer {
 	}
 	p = add(p, uintptr(i)*4)
 	off := *(*uint32)(p)
+	// Return off == ^uint32(0) ? 0 : f.datap.gofunc + uintptr(off), but without branches.
+	// The compiler calculates mask on most architectures using conditional assignment.
+	var mask uintptr
 	if off == ^uint32(0) {
-		return nil
+		mask = 1
 	}
-	return unsafe.Pointer(f.datap.gofunc + uintptr(off))
+	mask--
+	raw := f.datap.gofunc + uintptr(off)
+	return unsafe.Pointer(raw & mask)
 }
 
 // step advances to the next pc, value pair in the encoded table.