diff --git a/gas/ChangeLog b/gas/ChangeLog
index ff0f65cf00d..e217c8243a4 100644
--- a/gas/ChangeLog
+++ b/gas/ChangeLog
@@ -1,3 +1,7 @@
+2021-06-19  Alan Modra  <amodra@gmail.com>
+
+	* config/tc-z80.c (signed_overflow, unsigned_overflow): Avoid UB.
+
 2021-06-19  Alan Modra  <amodra@gmail.com>
 
 	* testsuite/gas/ppc/raw.s: Use 0 as pli constant.
diff --git a/gas/config/tc-z80.c b/gas/config/tc-z80.c
index 47d1405c2b6..303296b7dab 100644
--- a/gas/config/tc-z80.c
+++ b/gas/config/tc-z80.c
@@ -3703,14 +3703,14 @@ md_assemble (char *str)
 static int
 signed_overflow (signed long value, unsigned bitsize)
 {
-  signed long max = (signed long)(1UL << (bitsize-1));
-  return value < -max || value >= max;
+  signed long max = (signed long) ((1UL << (bitsize - 1)) - 1);
+  return value < -max - 1 || value > max;
 }
 
 static int
 unsigned_overflow (unsigned long value, unsigned bitsize)
 {
-  return (value >> bitsize) != 0;
+  return value >> (bitsize - 1) >> 1 != 0;
 }
 
 static int