diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index 1aba68c12737..e798936a40ab 100644
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,9 @@
+2007-02-01  Geoffrey Keating  <geoffk@apple.com>
+
+	* config/rs6000/rs6000.c (rs6000_stack_info): Correct
+	altivec_padding_size calculation on AIX.  Improve comment, add
+	assert to verify that it's right.
+
 2007-01-31  Anatoly Sokolov <aesok@post.ru>
 
 	PR target/19087
diff --git a/gcc/config/rs6000/rs6000.c b/gcc/config/rs6000/rs6000.c
index db59a57dc79f..54802a8849c1 100644
--- a/gcc/config/rs6000/rs6000.c
+++ b/gcc/config/rs6000/rs6000.c
@@ -13127,10 +13127,11 @@ rs6000_stack_info (void)
 	  info_ptr->vrsave_save_offset
 	    = info_ptr->gp_save_offset - info_ptr->vrsave_size;
 
-	  /* Align stack so vector save area is on a quadword boundary.  */
+	  /* Align stack so vector save area is on a quadword boundary.  
+	     The padding goes above the vectors.  */
 	  if (info_ptr->altivec_size != 0)
 	    info_ptr->altivec_padding_size
-	      = (-info_ptr->vrsave_save_offset) % 16;
+	      = info_ptr->vrsave_save_offset & 0xF;
 	  else
 	    info_ptr->altivec_padding_size = 0;
 
@@ -13138,6 +13139,8 @@ rs6000_stack_info (void)
 	    = info_ptr->vrsave_save_offset
 	    - info_ptr->altivec_padding_size
 	    - info_ptr->altivec_size;
+	  gcc_assert (info_ptr->altivec_size == 0
+		      || info_ptr->altivec_save_offset % 16 == 0);
 
 	  /* Adjust for AltiVec case.  */
 	  info_ptr->ehrd_offset = info_ptr->altivec_save_offset - ehrd_size;