Add a gen_int_shift_amount helper function

This patch adds a helper routine that constructs rtxes
for constant shift amounts, given the mode of the value
being shifted.  As well as helping with the SVE patches, this
is one step towards allowing CONST_INTs to have a real mode.

One long-standing problem has been to decide what the mode
of a shift count should be for arbitrary rtxes (as opposed to those
directly tied to a target pattern).  Realistic choices would be
the mode of the shifted elements, word_mode, QImode, or the same
mode as the shift optabs (in which case what should the mode
be when the target doesn't have a pattern?)

For now the patch picks the mode of the shifted elements,
but with a ??? comment.

2017-11-06  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* emit-rtl.h (gen_int_shift_amount): Declare.
	* emit-rtl.c (gen_int_shift_amount): New function.
	* asan.c (asan_emit_stack_protection): Use gen_int_shift_amount
	instead of GEN_INT.
	* calls.c (shift_return_value): Likewise.
	* cse.c (fold_rtx): Likewise.
	* dse.c (find_shift_sequence): Likewise.
	* expmed.c (init_expmed_one_mode, store_bit_field_1, expand_shift_1)
	(expand_shift, expand_smod_pow2): Likewise.
	* lower-subreg.c (shift_cost): Likewise.
	* optabs.c (expand_superword_shift, expand_doubleword_mult)
	(expand_unop, expand_binop, shift_amt_for_vec_perm_mask)
	(expand_vec_perm_var): Likewise.
	* simplify-rtx.c (simplify_unary_operation_1): Likewise.
	(simplify_binary_operation_1): Likewise.
	* combine.c (try_combine, find_split_point, force_int_to_mode)
	(simplify_shift_const_1, simplify_shift_const): Likewise.
	(change_zero_ext): Likewise.  Use simplify_gen_binary.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>

From-SVN: r255745

1 files changed, 15 insertions, 0 deletions

diff --git a/gcc/emit-rtl.c b/gcc/emit-rtl.c
index 5ab1fb7..0012aa7 100644
--- a/gcc/emit-rtl.c
+++ b/gcc/emit-rtl.c
@@ -6418,6 +6418,21 @@ need_atomic_barrier_p (enum memmodel model, bool pre)
     }
 }
 
+/* Return a constant shift amount for shifting a value of mode MODE
+   by VALUE bits.  */
+
+rtx
+gen_int_shift_amount (machine_mode, HOST_WIDE_INT value)
+{
+  /* Try to use a 64-bit mode, to avoid any truncation, but honor
+     MAX_FIXED_MODE_SIZE if that's smaller.
+
+     ??? Perhaps this should be automatically derived from the .md files
+     instead, or perhaps have a target hook.  */
+  scalar_int_mode shift_mode = int_mode_for_size (64, 1).require ();
+  return gen_int_mode (value, shift_mode);
+}
+
 /* Initialize fields of rtl_data related to stack alignment.  */
 
 void