[PATCH] rs6000: Fix default alignment ABI break caused by MMA base support
Peter Bergner
bergner@linux.ibm.com
Fri Nov 6 22:18:00 GMT 2020
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
bits 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
(GLIBC is going to fix that too). The fix in GCC is to revert the
BIGGEST_ALIGNMENT change and to force the alignment on the type itself
rather than the mode used by the type.
This passes bootstrap and regtesting on powerpc64le-linux with no regressions.
Ok for mainline and the GCC 10 release branch after some burn in?
Peter
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.
diff --git a/gcc/config/rs6000/rs6000.h b/gcc/config/rs6000/rs6000.h
index bbd8060e143..5a47aa14722 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/config/rs6000/rs6000-call.c b/gcc/config/rs6000/rs6000-call.c
index 92378e958a9..3bd89a79bad 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/testsuite/gcc.target/powerpc/mma-alignment.c b/gcc/testsuite/gcc.target/powerpc/mma-alignment.c
new file mode 100644
index 00000000000..09818931b48
--- /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;
+}
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