cmd/compile: replace magic numbers "2" and "1" with named constant

This was originally done for a #next-encoding-based check for
misbehaving loops, but it's a good idea anyhow because it makes
the code slightly easier to follow or change (we may decide to
check for errors the "other way" anyhow, later).

Change-Id: I2ba8f6e0f9146f0ff148a900eabdefd0fffebf8b
Reviewed-on: https://go-review.googlesource.com/c/go/+/540261
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

1 files changed, 10 insertions, 3 deletions

diff --git a/src/cmd/compile/internal/rangefunc/rewrite.go b/src/cmd/compile/internal/rangefunc/rewrite.go
index ac12c53c2b..c28c9a1207 100644
--- a/src/cmd/compile/internal/rangefunc/rewrite.go
+++ b/src/cmd/compile/internal/rangefunc/rewrite.go
@@ -640,6 +640,13 @@ func (r *rewriter) editReturn(x *syntax.ReturnStmt) syntax.Stmt {
 	return bl
 }
 
+// perLoopStep is part of the encoding of loop-spanning control flow
+// for function range iterators.  Each multiple of two encodes a "return false"
+// passing control to an enclosing iterator; a terminal value of 1 encodes
+// "return true" (i.e., local continue) from the body function, and a terminal
+// value of 0 encodes executing the remainder of the body function.
+const perLoopStep = 2
+
 // editBranch returns the replacement for the branch statement x,
 // or x itself if it should be left alone.
 // See the package doc comment above for more context.
@@ -734,7 +741,7 @@ func (r *rewriter) editBranch(x *syntax.BranchStmt) syntax.Stmt {
 
 		// Set next to break the appropriate number of times;
 		// the final time may be a continue, not a break.
-		next = 2 * depth
+		next = perLoopStep * depth
 		if x.Tok == syntax.Continue {
 			next--
 		}
@@ -948,10 +955,10 @@ func (r *rewriter) checks(loop *forLoop, pos syntax.Pos) []syntax.Stmt {
 			list = append(list, r.ifNext(syntax.Lss, 0, retStmt(r.useVar(r.false))))
 		}
 		if loop.checkBreak {
-			list = append(list, r.ifNext(syntax.Geq, 2, retStmt(r.useVar(r.false))))
+			list = append(list, r.ifNext(syntax.Geq, perLoopStep, retStmt(r.useVar(r.false))))
 		}
 		if loop.checkContinue {
-			list = append(list, r.ifNext(syntax.Eql, 1, retStmt(r.useVar(r.true))))
+			list = append(list, r.ifNext(syntax.Eql, perLoopStep-1, retStmt(r.useVar(r.true))))
 		}
 	}