cmd/compile: more minor cleanup in shortcircuitBlock

Continue to simplify, rename for clarity,
improve docs, and reduce variable scope.

This is in preparation for this function becoming
more complicated.

Passes toolstash-check.

Updates #37608

Change-Id: I630a4e07c92297c46d18aea69ec29852d6371ff0
Reviewed-on: https://go-review.googlesource.com/c/go/+/222919
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

diff --git a/src/cmd/compile/internal/ssa/shortcircuit.go b/src/cmd/compile/internal/ssa/shortcircuit.go
index 274ef9a1283d05c6744f441a4fb0f79b4a267856..cc23701c3959440d4a430219560a1a274754c389 100644 (file)
--- a/src/cmd/compile/internal/ssa/shortcircuit.go
+++ b/src/cmd/compile/internal/ssa/shortcircuit.go
@@ -105,10 +105,10 @@ func shortcircuitBlock(b *Block) bool {
        // Track the negations so that we can swap successors as needed later.
        ctl := b.Controls[0]
        nval := 1 // the control value
-       swap := false
+       var swap int64
        for ctl.Uses == 1 && ctl.Block == b && (ctl.Op == OpCopy || ctl.Op == OpNot) {
                if ctl.Op == OpNot {
-                       swap = !swap
+                       swap = 1 ^ swap
                }
                ctl = ctl.Args[0]
                nval++ // wrapper around control value
@@ -129,25 +129,19 @@ func shortcircuitBlock(b *Block) bool {
                return false
        }
 
-       a := ctl.Args[cidx]
-       // The predecessor we come in from.
-       e1 := b.Preds[cidx]
-       p := e1.b
-       pi := e1.i
+       // p is the predecessor corresponding to cidx.
+       pe := b.Preds[cidx]
+       p := pe.b
+       pi := pe.i
 
-       // The successor we always go to when coming in
-       // from that predecessor.
-       si := 1 - a.AuxInt
-       if swap {
-               si = 1 - si
-       }
-       e2 := b.Succs[si]
-       t := e2.b
+       // t is the "taken" branch: the successor we always go to when coming in from p.
+       ti := 1 ^ ctl.Args[cidx].AuxInt ^ swap
+       te := b.Succs[ti]
+       t := te.b
        if p == b || t == b {
                // This is an infinite loop; we can't remove it. See issue 33903.
                return false
        }
-       ti := e2.i
 
        // We're committed. Update CFG and Phis.
 
@@ -164,11 +158,11 @@ func shortcircuitBlock(b *Block) bool {
 
        // Fix up t to have one more predecessor.
        t.Preds = append(t.Preds, Edge{p, pi})
-       for _, w := range t.Values {
-               if w.Op != OpPhi {
+       for _, v := range t.Values {
+               if v.Op != OpPhi {
                        continue
                }
-               w.AddArg(w.Args[ti])
+               v.AddArg(v.Args[te.i])
        }
 
        if len(b.Preds) == 0 {