master: Fix ash-modfx constant folding

The branch "master" has been updated in SBCL:
       via  186fa8ba9f2245fd507f97208104c71d03ce7afe (commit)
      from  df956be42674804c273fabaa4643681282fb0273 (commit)

- Log -----------------------------------------------------------------
commit 186fa8ba9f2245fd507f97208104c71d03ce7afe
Author: Stas Boukarev <[email protected]>
Date:   Thu Apr 9 04:12:08 2026 +0300

    Fix ash-modfx constant folding
---
 src/code/numbers.lisp   |  6 ++----
 tests/arith-2.pure.lisp | 10 ++++++++++
 2 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/src/code/numbers.lisp b/src/code/numbers.lisp
index e8f67efc6..6f8f8bc87 100644
--- a/src/code/numbers.lisp
+++ b/src/code/numbers.lisp
@@ -1983,7 +1983,7 @@ and the number of 0 bits if INTEGER is negative."
 
 #+(or x86 x86-64 arm arm64)
 (defun sb-vm::ash-left-modfx (integer amount)
-  (let ((fixnum-width (- sb-vm:n-word-bits sb-vm:n-fixnum-tag-bits)))
+  (let ((fixnum-width sb-vm:n-fixnum-bits))
     (etypecase integer
       (fixnum (sb-c::mask-signed-field fixnum-width (ash integer amount)))
       (integer (sb-c::mask-signed-field fixnum-width (ash (sb-c::mask-signed-field fixnum-width integer) amount))))))
@@ -1993,9 +1993,7 @@ and the number of 0 bits if INTEGER is negative."
     (ldb (byte 64 0) (ash integer amount)))
 
   (defun sb-vm::ash-modfx (integer amount)
-    (if (minusp integer)
-        (sb-c::mask-signed-field sb-vm:n-fixnum-bits (ash integer amount))
-        (logand most-positive-fixnum (ash integer amount)))))
+    (sb-c::mask-signed-field sb-vm:n-fixnum-bits (ash integer amount))))
 
 (defun sb-vm::truncate-mod64 (a b)
   (multiple-value-bind (q r) (truncate a b)
diff --git a/tests/arith-2.pure.lisp b/tests/arith-2.pure.lisp
index 7c6ee9bb0..ed480fac7 100644
--- a/tests/arith-2.pure.lisp
+++ b/tests/arith-2.pure.lisp
@@ -2507,3 +2507,13 @@
               (integer b))
      (truncate a b))
    (values (integer -5 5) (rational -5 4) &optional)))
+
+(with-test (:name :ash-modfx-constant-fold)
+  (checked-compile-and-assert
+      ()
+      `(lambda (b)
+         (declare (fixnum b))
+         (when (eq b (1- sb-vm:n-fixnum-bits))
+           (ldb (byte sb-vm:n-fixnum-bits 0) (ash 1 b))))
+    (((1- sb-vm:n-fixnum-bits)) (ash 1 (1- sb-vm:n-fixnum-bits)))
+    ((0) nil)))

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