Thanks for the comments! Here is a second go-round at the patch with
improved documentation. I'm happy to change the wording if it can be
further improved.
Thanks,
Bill
2014-01-09 Bill Schmidt <wschm...@linux.vnet.ibm.com>
* doc/invoke.texi: Add -maltivec={be,le} options, and document
default element-order behavior for -maltivec.
* config/rs6000/rs6000.opt: Add -maltivec={be,le} options.
* config/rs6000/rs6000.c (rs6000_option_override_internal): Ensure
that -maltivec={le,be} implies -maltivec; disallow -maltivec=le
when targeting big endian, at least for now.
* config/rs6000/rs6000.h: Add #define of VECTOR_ELT_ORDER_BIG.
Index: gcc/doc/invoke.texi
===================================================================
--- gcc/doc/invoke.texi (revision 206442)
+++ gcc/doc/invoke.texi (working copy)
@@ -18855,6 +18855,37 @@ the AltiVec instruction set. You may also need to
@option{-mabi=altivec} to adjust the current ABI with AltiVec ABI
enhancements.
+When -maltivec is used, rather than -maltivec=le or -maltivec=be, the
+element order for Altivec intrinsics such as vec_splat, vec_extract,
+and vec_insert will match array element order corresponding to the
+endianness of the target. That is, element zero identifies the
+leftmost element in a vector register when targeting a big-endian
+platform, and identifies the rightmost element in a vector register
+when targeting a little-endian platform.
+
+@item -maltivec=be
+@opindex maltivec=be
+Generate Altivec instructions using big-endian element order,
+regardless of whether the target is big- or little-endian. This is
+the default when targeting a big-endian platform.
+
+The element order is used to interpret element numbers in Altivec
+intrinsics such as vec_splat, vec_extract, and vec_insert. By
+default, these will match array element order corresponding to the
+endianness for the target.
+
+@item -maltivec=le
+@opindex maltivec=le
+Generate Altivec instructions using little-endian element order,
+regardless of whether the target is big- or little-endian. This is
+the default when targeting a little-endian platform. This option is
+currently ignored when targeting a big-endian platform.
+
+The element order is used to interpret element numbers in Altivec
+intrinsics such as vec_splat, vec_extract, and vec_insert. By
+default, these will match array element order corresponding to the
+endianness for the target.
+
@item -mvrsave
@itemx -mno-vrsave
@opindex mvrsave
Index: gcc/config/rs6000/rs6000.opt
===================================================================
--- gcc/config/rs6000/rs6000.opt (revision 206442)
+++ gcc/config/rs6000/rs6000.opt (working copy)
@@ -140,6 +140,14 @@ maltivec
Target Report Mask(ALTIVEC) Var(rs6000_isa_flags)
Use AltiVec instructions
+maltivec=le
+Target Report RejectNegative Var(rs6000_altivec_element_order, 1) Save
+Generate Altivec instructions using little-endian element order
+
+maltivec=be
+Target Report RejectNegative Var(rs6000_altivec_element_order, 2)
+Generate Altivec instructions using big-endian element order
+
mhard-dfp
Target Report Mask(DFP) Var(rs6000_isa_flags)
Use decimal floating point instructions
Index: gcc/config/rs6000/rs6000.c
===================================================================
--- gcc/config/rs6000/rs6000.c (revision 206442)
+++ gcc/config/rs6000/rs6000.c (working copy)
@@ -3238,6 +3238,18 @@ rs6000_option_override_internal (bool global_init_
&& !(processor_target_table[tune_index].target_enable & OPTION_MASK_HTM))
rs6000_isa_flags |= ~rs6000_isa_flags_explicit & OPTION_MASK_STRICT_ALIGN;
+ /* -maltivec={le,be} implies -maltivec. */
+ if (rs6000_altivec_element_order != 0)
+ rs6000_isa_flags |= OPTION_MASK_ALTIVEC;
+
+ /* Disallow -maltivec=le in big endian mode for now. This is not
+ known to be useful for anyone. */
+ if (BYTES_BIG_ENDIAN && rs6000_altivec_element_order == 1)
+ {
+ warning (0, N_("-maltivec=le not allowed for big-endian targets"));
+ rs6000_altivec_element_order = 0;
+ }
+
/* Add some warnings for VSX. */
if (TARGET_VSX)
{
Index: gcc/config/rs6000/rs6000.h
===================================================================
--- gcc/config/rs6000/rs6000.h (revision 206442)
+++ gcc/config/rs6000/rs6000.h (working copy)
@@ -468,6 +468,15 @@ extern int rs6000_vector_align[];
? rs6000_vector_align[(MODE)] \
: (int)GET_MODE_BITSIZE ((MODE)))
+/* Determine the element order to use for vector instructions. By
+ default we use big-endian element order when targeting big-endian,
+ and little-endian element order when targeting little-endian. For
+ programs being ported from BE Power to LE Power, it can sometimes
+ be useful to use big-endian element order when targeting little-endian.
+ This is set via -maltivec=be, for example. */
+#define VECTOR_ELT_ORDER_BIG \
+ (BYTES_BIG_ENDIAN || (rs6000_altivec_element_order == 2))
+
/* Alignment options for fields in structures for sub-targets following
AIX-like ABI.
ALIGN_POWER word-aligns FP doubles (default AIX ABI).
On Wed, 2014-01-08 at 16:46 -0500, David Edelsohn wrote:
> On Tue, Jan 7, 2014 at 6:59 PM, Bill Schmidt
> <wschm...@linux.vnet.ibm.com> wrote:
> > On Tue, 2014-01-07 at 22:18 +0000, Joseph S. Myers wrote:
> >> On Tue, 7 Jan 2014, Bill Schmidt wrote:
> >>
> >> > Yes, sorry for not being more clear. This is indeed for interpretation
> >> > of element numbers in Altivec intrinsics such as vec_splat, vec_extract,
> >> > vec_insert, and so forth. By default these will match array element
> >> > order for the target endianness. But with -maltivec=be for a little
> >> > endian target, we will force use of big-endian element order (matching
> >> > the behavior of the underlying hardware instructions).
> >>
> >> Thanks for the explanation. I think you should make the .texi
> >> documentation say something more like this.
> >>
> >
> > Sure, I can wordsmith something along those lines. Thanks for the
> > feedback!
>
> This patch is okay with the documentation clarification requested by Joseph.
>
> I also would suggest removing "but may be enabled in the future" from
> the "le" option and limit the comment to ignored on big-endian
> targets.
>
> Also, please add a comment to -maltivec that it defaults to the native
> endian order. And for -maltivec=be, please state that this is the
> default for big-endian; for -maltivec=le, please state that this is
> the default for little-endian. It's important to be clear and
> redundant in this type of documentation.
>
> Thanks, David
>
