Eric Botcazou <ebotca...@adacore.com> writes:
>> OK, how about:
>>
>> /* ??? For historical reasons, reject modes that would normally
>> receive greater alignment, even if unaligned accesses are
>> acceptable. This has both advantages and disadvantages.
>
> Fine with me.
>
>> I think here we really do want unit (i.e. the GET_MODE_BITSIZE).
>> We're dividing the bitfield into unit-sized chunks and want to know
>> whether those chunks are aligned or not. If they aren't aligned,
>> we need to know whether unaligned accesses are OK.
>
> We could conceivably have an aligned mode with bitsize 32 and alignment 16
> (i.e. STRICT_ALIGNMENT 1 and BIGGEST_ALIGNMENT 16), meaning that we can use
> 16-bit aligned 32-bit chunks in this mode. Why would the bitsize matter
> (assuming that it's a multiple of the alignment)?
I suppose I was putting too much store by the expmed.c code. How does
this version look? Tested on x86_64-linux-gnu, powerpc64-linux-gnu
and cris-elf.
Richard
gcc/
PR middle-end/55438
* expmed.c (simple_mem_bitfield_p): New function, extracted from
store_bit_field_1 and extract_bit_field_1. Use GET_MODE_ALIGNMENT
rather than bitsize when checking the alignment.
* stor-layout.c (bit_field_mode_iterator::bit_field_mode_iterator):
Don't limit ALIGN_. Assume that memory is mapped in chunks of at
least word size, regardless of BIGGEST_ALIGNMENT.
(bit_field_mode_iterator::get_mode): Use GET_MODE_ALIGNMENT rather
than unit when checking the alignment.
(get_best_mode): Use GET_MODE_ALIGNMENT.
Index: gcc/expmed.c
===================================================================
--- gcc/expmed.c 2012-11-20 19:49:07.000000000 +0000
+++ gcc/expmed.c 2012-11-27 17:27:38.381139339 +0000
@@ -416,6 +416,21 @@ lowpart_bit_field_p (unsigned HOST_WIDE_
else
return bitnum % BITS_PER_WORD == 0;
}
+
+/* Return true if OP is a memory and if a bitfield of size BITSIZE at
+ bit number BITNUM can be treated as a simple value of mode MODE. */
+
+static bool
+simple_mem_bitfield_p (rtx op0, unsigned HOST_WIDE_INT bitsize,
+ unsigned HOST_WIDE_INT bitnum, enum machine_mode mode)
+{
+ return (MEM_P (op0)
+ && bitnum % BITS_PER_UNIT == 0
+ && bitsize == GET_MODE_BITSIZE (mode)
+ && (!SLOW_UNALIGNED_ACCESS (mode, MEM_ALIGN (op0))
+ || (bitnum % GET_MODE_ALIGNMENT (mode) == 0
+ && MEM_ALIGN (op0) % GET_MODE_ALIGNMENT (mode) == 0)));
+}
/* Try to use instruction INSV to store VALUE into a field of OP0.
BITSIZE and BITNUM are as for store_bit_field. */
@@ -624,12 +639,7 @@ store_bit_field_1 (rtx str_rtx, unsigned
/* If the target is memory, storing any naturally aligned field can be
done with a simple store. For targets that support fast unaligned
memory, any naturally sized, unit aligned field can be done directly. */
- if (MEM_P (op0)
- && bitnum % BITS_PER_UNIT == 0
- && bitsize == GET_MODE_BITSIZE (fieldmode)
- && (!SLOW_UNALIGNED_ACCESS (fieldmode, MEM_ALIGN (op0))
- || (bitnum % bitsize == 0
- && MEM_ALIGN (op0) % bitsize == 0)))
+ if (simple_mem_bitfield_p (op0, bitsize, bitnum, fieldmode))
{
op0 = adjust_bitfield_address (op0, fieldmode, bitnum / BITS_PER_UNIT);
emit_move_insn (op0, value);
@@ -1454,12 +1464,7 @@ extract_bit_field_1 (rtx str_rtx, unsign
/* Extraction of a full MODE1 value can be done with a load as long as
the field is on a byte boundary and is sufficiently aligned. */
- if (MEM_P (op0)
- && bitnum % BITS_PER_UNIT == 0
- && bitsize == GET_MODE_BITSIZE (mode1)
- && (!SLOW_UNALIGNED_ACCESS (mode1, MEM_ALIGN (op0))
- || (bitnum % bitsize == 0
- && MEM_ALIGN (op0) % bitsize == 0)))
+ if (simple_mem_bitfield_p (op0, bitsize, bitnum, mode1))
{
op0 = adjust_bitfield_address (op0, mode1, bitnum / BITS_PER_UNIT);
return convert_extracted_bit_field (op0, mode, tmode, unsignedp);
Index: gcc/stor-layout.c
===================================================================
--- gcc/stor-layout.c 2012-11-24 12:16:28.947902915 +0000
+++ gcc/stor-layout.c 2012-11-26 23:16:36.273308430 +0000
@@ -2643,15 +2643,17 @@ fixup_unsigned_type (tree type)
unsigned int align, bool volatilep)
: mode_ (GET_CLASS_NARROWEST_MODE (MODE_INT)), bitsize_ (bitsize),
bitpos_ (bitpos), bitregion_start_ (bitregion_start),
- bitregion_end_ (bitregion_end), align_ (MIN (align, BIGGEST_ALIGNMENT)),
+ bitregion_end_ (bitregion_end), align_ (align),
volatilep_ (volatilep), count_ (0)
{
if (!bitregion_end_)
{
- /* We can assume that any aligned chunk of ALIGN_ bits that overlaps
+ /* We can assume that any aligned chunk of UNITS bits that overlaps
the bitfield is mapped and won't trap. */
- bitregion_end_ = bitpos + bitsize + align_ - 1;
- bitregion_end_ -= bitregion_end_ % align_ + 1;
+ unsigned HOST_WIDE_INT units = MIN (align, MAX (BIGGEST_ALIGNMENT,
+ BITS_PER_WORD));
+ bitregion_end_ = bitpos + bitsize + units - 1;
+ bitregion_end_ -= bitregion_end_ % units + 1;
}
}
@@ -2694,7 +2696,8 @@ bit_field_mode_iterator::next_mode (enum
break;
/* Stop if the mode requires too much alignment. */
- if (unit > align_ && SLOW_UNALIGNED_ACCESS (mode_, align_))
+ if (GET_MODE_ALIGNMENT (mode_) > align_
+ && SLOW_UNALIGNED_ACCESS (mode_, align_))
break;
*out_mode = mode_;
@@ -2753,8 +2756,9 @@ get_best_mode (int bitsize, int bitpos,
enum machine_mode widest_mode = VOIDmode;
enum machine_mode mode;
while (iter.next_mode (&mode)
- /* ??? For historical reasons, reject modes that are wider than
- the alignment. This has both advantages and disadvantages.
+ /* ??? For historical reasons, reject modes that would normally
+ receive greater alignment, even if unaligned accesses are
+ acceptable. This has both advantages and disadvantages.
Removing this check means that something like:
struct s { unsigned int x; unsigned int y; };
@@ -2808,7 +2812,7 @@ get_best_mode (int bitsize, int bitpos,
causes store_bit_field to keep a 128-bit memory reference,
so that the final bitfield reference still has a MEM_EXPR
and MEM_OFFSET. */
- && GET_MODE_BITSIZE (mode) <= align
+ && GET_MODE_ALIGNMENT (mode) <= align
&& (largest_mode == VOIDmode
|| GET_MODE_SIZE (mode) <= GET_MODE_SIZE (largest_mode)))
{
