rs6000: Fix default alignment ABI break caused by MMA base support

As part of the MMA base support, we incremented BIGGEST_ALIGNMENT in
order to align the __vector_pair and __vector_quad types to 256 and 512
bytes respectively.  This had the unintended effect of changing the
default alignment used by __attribute__ ((__aligned__)) which causes
an ABI break because of some dodgy code in GLIBC's struct pthread.
The fix is to revert the BIGGEST_ALIGNMENT change and to force the
alignment on the type itself rather than the mode used by the type.

2020-11-06  Peter Bergner  <bergner@linux.ibm.com>

gcc/
	* config/rs6000/rs6000.h (BIGGEST_ALIGNMENT): Revert previous commit
	so as not to break the ABI.
	* config/rs6000/rs6000-call.c (rs6000_init_builtins): Set the ABI
	mandated alignment for __vector_pair and __vector_quad types.

gcc/testsuite/
	* gcc.target/powerpc/mma-alignment.c: New test.

3 files changed, 47 insertions, 2 deletions

diff --git a/gcc/config/rs6000/rs6000-call.c b/gcc/config/rs6000/rs6000-call.c
index 92378e9..3bd89a7 100644
--- a/gcc/config/rs6000/rs6000-call.c
+++ b/gcc/config/rs6000/rs6000-call.c
@@ -13191,12 +13191,14 @@ rs6000_init_builtins (void)
   if (TARGET_EXTRA_BUILTINS)
     {
       vector_pair_type_node = make_unsigned_type (256);
+      SET_TYPE_ALIGN (vector_pair_type_node, 256);
       SET_TYPE_MODE (vector_pair_type_node, POImode);
       layout_type (vector_pair_type_node);
       lang_hooks.types.register_builtin_type (vector_pair_type_node,
 					      "__vector_pair");
 
       vector_quad_type_node = make_unsigned_type (512);
+      SET_TYPE_ALIGN (vector_quad_type_node, 512);
       SET_TYPE_MODE (vector_quad_type_node, PXImode);
       layout_type (vector_quad_type_node);
       lang_hooks.types.register_builtin_type (vector_quad_type_node,
diff --git a/gcc/config/rs6000/rs6000.h b/gcc/config/rs6000/rs6000.h
index bbd8060..5a47aa1 100644
--- a/gcc/config/rs6000/rs6000.h
+++ b/gcc/config/rs6000/rs6000.h
@@ -776,8 +776,10 @@ extern unsigned rs6000_pointer_size;
 /* Allocation boundary (in *bits*) for the code of a function.  */
 #define FUNCTION_BOUNDARY 32
 
-/* No data type wants to be aligned rounder than this.  */
-#define BIGGEST_ALIGNMENT (TARGET_MMA ? 512 : 128)
+/* No data type is required to be aligned rounder than this.  Warning, if
+   BIGGEST_ALIGNMENT is changed, then this may be an ABI break.  An example
+   of where this can break an ABI is in GLIBC's struct _Unwind_Exception.  */
+#define BIGGEST_ALIGNMENT 128
 
 /* Alignment of field after `int : 0' in a structure.  */
 #define EMPTY_FIELD_BOUNDARY 32
diff --git a/gcc/testsuite/gcc.target/powerpc/mma-alignment.c b/gcc/testsuite/gcc.target/powerpc/mma-alignment.c
new file mode 100644
index 0000000..0981893
--- /dev/null
+++ b/gcc/testsuite/gcc.target/powerpc/mma-alignment.c
@@ -0,0 +1,41 @@
+/* { dg-do run } */
+/* { dg-require-effective-target hard_float } */
+/* { dg-options "-O2 -mhard-float" } */
+
+#include <stdlib.h>
+
+/* The MMA types below are enabled for pre-power10 compiles, because the
+   built-ins that use them must always be initialized in case the user has
+   a target attribute or pragma on a function that uses the MMA built-ins.
+   Since the test below doesn't need any other MMA support, we can enable
+   this test case on basically any cpu that has hard floating point
+   registers.  */
+
+struct
+{
+  int __attribute__ ((__aligned__)) ivar;
+  __vector_pair pair;
+  __vector_quad quad;
+} s;
+
+int
+main (void)
+{
+  /* Verify default alignment is 16-byte aligned (BIGGEST_ALIGNMENT).
+     This may change in the future, but that is an ABI break, so this
+     hardcoded test case is here to be a noisy FAIL as a warning, in
+     case the ABI change was unintended and unwanted.  An example of where
+     this can break an ABI is in glibc's struct _Unwind_Exception.  */
+  if (__alignof__ (s.ivar) != 16)
+    abort ();
+
+  /* Verify __vector_pair types are 32-byte aligned.  */
+  if (__alignof__ (s.pair) != 32)
+    abort ();
+
+  /* Verify __vector_quad types are 64-byte aligned.  */
+  if (__alignof__ (s.quad) != 64)
+    abort ();
+
+  return 0;
+}