go/printer: use architecture-independent math for alignment decisions

The `exprList` function uses a geometric mean of expression sizes to
decide whether to break column alignment in composite literals.
Previously, this was computed using `math.Exp` and `math.Log`,
which are not guaranteed to produce bit-for-bit identical results
across CPU architectures. For example, `math.Exp(math.Log(95))`
returns exactly 95.0 on arm64 but 94.999... on amd64, which can flip
the alignment decision.

Replace math operations with approximations that are
architecture-independent. The approximations are sufficient
for this heuristic and are guaranteed to produce identical formatting
on all architectures.

Fixes #77959

Change-Id: I78d00d85fd62859803d14d619b4a45e2e5081499
Reviewed-on: https://go-review.googlesource.com/c/go/+/752862
Auto-Submit: Alan Donovan <adonovan@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

4 files changed, 48 insertions, 7 deletions

diff --git a/src/go/printer/math.go b/src/go/printer/math.go
new file mode 100644
index 0000000000..543a9e99d5
--- /dev/null
+++ b/src/go/printer/math.go
@@ -0,0 +1,28 @@
+// Copyright 2026 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package printer
+
+import "math"
+
+// log2ish returns a crude approximation to log₂(x).
+// The result is only used for heuristic alignment decisions and should
+// not be used where precision matters.
+// The approximation is guaranteed to produce identical results
+// across all architectures.
+func log2ish(x float64) float64 {
+	f, e := math.Frexp(x)
+	return float64(e) + 2*(f-1)
+}
+
+// exp2ish returns a crude approximation to 2**x.
+// The result is only used for heuristic alignment decisions and should
+// not be used where precision matters.
+// The approximation is guaranteed to produce identical results
+// across all architectures.
+func exp2ish(x float64) float64 {
+	n := math.Floor(x)
+	f := x - n
+	return math.Ldexp(1+f, int(n))
+}
diff --git a/src/go/printer/nodes.go b/src/go/printer/nodes.go
index a295a68d6f..6c7a2a031a 100644
--- a/src/go/printer/nodes.go
+++ b/src/go/printer/nodes.go
@@ -11,7 +11,6 @@ package printer
 import (
 	"go/ast"
 	"go/token"
-	"math"
 	"strconv"
 	"strings"
 	"unicode"
@@ -183,9 +182,9 @@ func (p *printer) exprList(prev0 token.Pos, list []ast.Expr, depth int, mode exp
 
 	// We use the ratio between the geometric mean of the previous key sizes and
 	// the current size to determine if there should be a break in the alignment.
-	// To compute the geometric mean we accumulate the ln(size) values (lnsum)
+	// To compute the geometric mean we accumulate the log₂(size) values (log2sum)
 	// and the number of sizes included (count).
-	lnsum := 0.0
+	log2sum := 0.0
 	count := 0
 
 	// print all list elements
@@ -228,8 +227,8 @@ func (p *printer) exprList(prev0 token.Pos, list []ast.Expr, depth int, mode exp
 			if count == 0 || prevSize <= smallSize && size <= smallSize {
 				useFF = false
 			} else {
-				const r = 2.5                               // threshold
-				geomean := math.Exp(lnsum / float64(count)) // count > 0
+				const r = 2.5                                // threshold
+				geomean := exp2ish(log2sum / float64(count)) // count > 0
 				ratio := float64(size) / geomean
 				useFF = r*ratio <= 1 || r <= ratio
 			}
@@ -260,7 +259,7 @@ func (p *printer) exprList(prev0 token.Pos, list []ast.Expr, depth int, mode exp
 				// the section), reset the geomean variables since we are
 				// starting a new group of elements with the next element.
 				if nbreaks > 1 {
-					lnsum = 0
+					log2sum = 0
 					count = 0
 				}
 			}
@@ -284,7 +283,7 @@ func (p *printer) exprList(prev0 token.Pos, list []ast.Expr, depth int, mode exp
 		}
 
 		if size > 0 {
-			lnsum += math.Log(float64(size))
+			log2sum += log2ish(float64(size))
 			count++
 		}
 
diff --git a/src/go/printer/testdata/alignment.golden b/src/go/printer/testdata/alignment.golden
index 96086ed906..74223e9856 100644
--- a/src/go/printer/testdata/alignment.golden
+++ b/src/go/printer/testdata/alignment.golden
@@ -170,3 +170,10 @@ var _ = S{
 	F1____:	[]string{},
 	F2:	[]string{},
 }
+
+// Issue #77959: formatting must be consistent across architectures.
+var _ = []any{
+	(expr_size_95_xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx)(0),	// 0
+	(expr_size_38_xxxxxxxxxxxxxxxxxxxx)(0),	// 1
+	(*expr_size_126_xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx)(nil),	// 2
+}
diff --git a/src/go/printer/testdata/alignment.input b/src/go/printer/testdata/alignment.input
index 323d2689e0..61c3ae2a98 100644
--- a/src/go/printer/testdata/alignment.input
+++ b/src/go/printer/testdata/alignment.input
@@ -177,3 +177,10 @@ var _ = S{
 	},
 	F2: []string{},
 }
+
+// Issue #77959: formatting must be consistent across architectures.
+var _ = []any{
+	(expr_size_95_xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx)(0), // 0
+	(expr_size_38_xxxxxxxxxxxxxxxxxxxx)(0), // 1
+	(*expr_size_126_xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx)(nil), // 2
+}