From bc5ffe5c79d45447648a012465e158f29ff5efa2 Mon Sep 17 00:00:00 2001
From: Joel Sing <joel@sing.id.au>
Date: Thu, 6 Nov 2025 01:05:29 +1100
Subject: internal/runtime/sys,math/bits: eliminate bounds checks on len8tab

The compiler cannot currently determine that the accesses to len8tab
are within bounds. Cast to uint8 to avoid unnecessary bounds checks.

Fixes #76166

Change-Id: I1fd930bba2b20d3998252c476308642e08ce00b4
Reviewed-on: https://go-review.googlesource.com/c/go/+/718040
Reviewed-by: Meng Zhuo <mengzhuo1203@gmail.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Auto-Submit: Joel Sing <joel@sing.id.au>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
---
 src/math/bits/bits.go | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

(limited to 'src/math')

diff --git a/src/math/bits/bits.go b/src/math/bits/bits.go
index 76ed1d03fc..6f9142f91a 100644
--- a/src/math/bits/bits.go
+++ b/src/math/bits/bits.go
@@ -317,7 +317,7 @@ func Len16(x uint16) (n int) {
 		x >>= 8
 		n = 8
 	}
-	return n + int(len8tab[x])
+	return n + int(len8tab[uint8(x)])
 }
 
 // Len32 returns the minimum number of bits required to represent x; the result is 0 for x == 0.
@@ -330,7 +330,7 @@ func Len32(x uint32) (n int) {
 		x >>= 8
 		n += 8
 	}
-	return n + int(len8tab[x])
+	return n + int(len8tab[uint8(x)])
 }
 
 // Len64 returns the minimum number of bits required to represent x; the result is 0 for x == 0.
@@ -347,7 +347,7 @@ func Len64(x uint64) (n int) {
 		x >>= 8
 		n += 8
 	}
-	return n + int(len8tab[x])
+	return n + int(len8tab[uint8(x)])
 }
 
 // --- Add with carry ---
-- 
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