emacs-27 6a4ed89: Fix replace-buffer-contents undefined behavior

[Paul Eggert]

branch: emacs-27
commit 6a4ed891d84b1ca1e4d68a6de9dd374f1fadd971
Author: Paul Eggert <eggert@cs.ucla.edu>
Commit: Paul Eggert <eggert@cs.ucla.edu>

    Fix replace-buffer-contents undefined behavior
    
    * src/editfns.c (Freplace_buffer_contents): Avoid undefined
    behavior with competing side effects in parallel subexpressions.
    Problem reported by Apple clang version 12.0.0 (clang-1200.0.32.29).
---
 src/editfns.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/src/editfns.c b/src/editfns.c
index 621e351..cd9633d 100644
--- a/src/editfns.c
+++ b/src/editfns.c
@@ -2053,6 +2053,8 @@ nil.  */)
      code.  */
   ptrdiff_t del_bytes = (size_t) size_a / CHAR_BIT + 1;
   ptrdiff_t ins_bytes = (size_t) size_b / CHAR_BIT + 1;
+  unsigned char *deletions = SAFE_ALLOCA (del_bytes);
+  unsigned char *insertions = SAFE_ALLOCA (ins_bytes);
   struct context ctx = {
     .buffer_a = a,
     .buffer_b = b,
@@ -2060,8 +2062,8 @@ nil.  */)
     .beg_b = min_b,
     .a_unibyte = BUF_ZV (a) == BUF_ZV_BYTE (a),
     .b_unibyte = BUF_ZV (b) == BUF_ZV_BYTE (b),
-    .deletions = SAFE_ALLOCA (del_bytes),
-    .insertions = SAFE_ALLOCA (ins_bytes),
+    .deletions = deletions,
+    .insertions = insertions,
     .fdiag = buffer + size_b + 1,
     .bdiag = buffer + diags + size_b + 1,
     .heuristic = true,