Richard Biener <richard.guent...@gmail.com> writes:
> On November 30, 2020 4:29:41 PM GMT+01:00, Richard Sandiford via Gcc-patches
> <gcc-patches@gcc.gnu.org> wrote:
>>dse.c:find_shift_sequence tries to represent a store and load
>>back as a shift right followed by a truncation. It therefore
>>needs to find an integer mode in which to do the shift right.
>>The loop it uses has the form:
>>
>> FOR_EACH_MODE_FROM (new_mode_iter,
>> smallest_int_mode_for_size (GET_MODE_BITSIZE (read_mode)))
>>
>>which implicitly assumes that read_mode has an equivalent integer mode.
>>As shown in the testcase, not all modes have such an integer mode.
>
> But if no such mode exists iterating won't help either? So why not simply
> fail when the mode does not exist?
You mean test against the maximum integer mode before the loop,
but keep the smallest_int_mode_for_size call as-is? I'm not sure
that's going to be more efficient in practice, and it seems less
obviously correct.
Alternatively, we could have a version of smallest_int_mode_for_size
that returns failure instead of asserting. But smallest_int_mode_for_size
is itself a FOR_EACH_MODE_* iterator, so it would iterate just as much as
the patch does.
smallest_int_mode_for_size also has some __int20 etc. handling that I
don't think we want here, and probably avoid by luck. (We already know
at this point that any shift value would be nonzero, which probably
weeds out most of the problematic cases for PSImode targets.)
find_shift_sequence already iterates over the modes itself and it
already has custom requirements in terms of when to break and when
to continue. It just seems simpler and more obvious for the loop to
iterate over all the modes directly rather than delegate part of the
iteration to another function.
Thanks,
Richard
>
>>This patch just makes the code start from the smallest integer mode and
>>skip modes that are too small. The loop already breaks at the first
>>mode wider than word_mode.
>>
>>Tested on aarch64-linux-gnu and x86_64-linux-gnu. OK for trunk and
>>GCC 10 branch?
>>
>>Richard
>>
>>
>>gcc/
>> PR rtl-optimization/98037
>> * dse.c (find_shift_sequence): Iterate over all integers and
>> skip modes that are too small.
>>
>>gcc/testsuite/
>> PR rtl-optimization/98037
>> * gcc.target/aarch64/sve/acle/general/pr98037.c: New test.
>>---
>> gcc/dse.c | 5 +++--
>> gcc/testsuite/gcc.target/aarch64/sve/acle/general/pr98037.c | 6 ++++++
>> 2 files changed, 9 insertions(+), 2 deletions(-)
>>create mode 100644
>>gcc/testsuite/gcc.target/aarch64/sve/acle/general/pr98037.c
>>
>>diff --git a/gcc/dse.c b/gcc/dse.c
>>index d65266b5476..651e6e7e71e 100644
>>--- a/gcc/dse.c
>>+++ b/gcc/dse.c
>>@@ -1757,8 +1757,7 @@ find_shift_sequence (poly_int64 access_size,
>> the machine. */
>>
>> opt_scalar_int_mode new_mode_iter;
>>- FOR_EACH_MODE_FROM (new_mode_iter,
>>- smallest_int_mode_for_size (GET_MODE_BITSIZE (read_mode)))
>>+ FOR_EACH_MODE_IN_CLASS (new_mode_iter, MODE_INT)
>> {
>> rtx target, new_reg, new_lhs;
>> rtx_insn *shift_seq, *insn;
>>@@ -1767,6 +1766,8 @@ find_shift_sequence (poly_int64 access_size,
>> new_mode = new_mode_iter.require ();
>> if (GET_MODE_BITSIZE (new_mode) > BITS_PER_WORD)
>> break;
>>+ if (maybe_lt (GET_MODE_SIZE (new_mode), GET_MODE_SIZE
>>(read_mode)))
>>+ continue;
>>
>> /* Try a wider mode if truncating the store mode to NEW_MODE
>> requires a real instruction. */
>>diff --git
>>a/gcc/testsuite/gcc.target/aarch64/sve/acle/general/pr98037.c
>>b/gcc/testsuite/gcc.target/aarch64/sve/acle/general/pr98037.c
>>new file mode 100644
>>index 00000000000..b91e940b18e
>>--- /dev/null
>>+++ b/gcc/testsuite/gcc.target/aarch64/sve/acle/general/pr98037.c
>>@@ -0,0 +1,6 @@
>>+/* { dg-options "-msve-vector-bits=1024 -O3" } */
>>+
>>+typedef __SVInt8_t vec __attribute__((arm_sve_vector_bits(1024)));
>>+struct pair { vec v[2]; };
>>+void use (struct pair *);
>>+vec f (struct pair p) { vec v = p.v[1]; use (&p); return v; }
