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Stack alignment on Darwin (PR78444)


Hi HJ,

I am trying to track down a misalignment of the stack on Darwin (pr78444).

In r163971 you applied this:

--- gcc/config/i386/darwin.h	(revision 163970)
+++ gcc/config/i386/darwin.h	(revision 163971)
@@ -79,7 +79,9 @@
    Failure to ensure this will lead to a crash in the system libraries
    or dynamic loader.  */
 #undef STACK_BOUNDARY
-#define STACK_BOUNDARY 128
+#define STACK_BOUNDARY \
+ ((profile_flag || (TARGET_64BIT && ix86_abi == MS_ABI)) \
+  ? 128 : BITS_PER_WORD)
 
 #undef MAIN_STACK_BOUNDARY
 #define MAIN_STACK_BOUNDARY 128
@@ -91,7 +93,7 @@
    it's below the minimum.  */
 #undef PREFERRED_STACK_BOUNDARY
 #define PREFERRED_STACK_BOUNDARY			\
-  MAX (STACK_BOUNDARY, ix86_preferred_stack_boundary)
+  MAX (128, ix86_preferred_stack_boundary

===

I realise it’s a long time ago …
.. but, have you any pointer to the reasoning here or what problem was being solved? 
(perhaps mail list discussion?)

===

Darwin’s 32b ABI mandates 128b alignment at functions calls:

 "The function calling conventions used in the IA-32 environment are the same as those used in the System V IA-32 ABI, with the following exceptions:
■ Different rules for returning structures
■ The stack is 16-byte aligned at the point of function calls
“

Darwin’s 64b ABI refers directly to the SysV document, which also mandates [section 3.2.2] 128b (or 256b when __m256 is passed).

===

The following patch resolves pr78444 - but it’s not clear if it’s a correct fix - or we should be looking for an alternate solution to whatever r193671 was intending to achieve.

thanks,
Iain

[PATCH] Fix for PR78444.

maybe.
---
 gcc/config/i386/i386.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/gcc/config/i386/i386.c b/gcc/config/i386/i386.c
index 163682bdff..405bfd082b 100644
--- a/gcc/config/i386/i386.c
+++ b/gcc/config/i386/i386.c
@@ -11530,6 +11530,15 @@ ix86_compute_frame_layout (void)
       crtl->preferred_stack_boundary = 128;
       crtl->stack_alignment_needed = 128;
     }
+  else if (TARGET_MACHO && crtl->preferred_stack_boundary < 128
+	   && !crtl->is_leaf)
+    {
+      /* Darwin's ABI specifies 128b alignment for both 32 and
+	 64 bit variants at call sites.  So we apply this if the
+	 current function is not a leaf.  */
+      crtl->preferred_stack_boundary = 128;
+      crtl->stack_alignment_needed = 128;
+    }
 
   stack_alignment_needed = crtl->stack_alignment_needed / BITS_PER_UNIT;
   preferred_alignment = crtl->preferred_stack_boundary / BITS_PER_UNIT;
-- 
2.17.1



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