Re: [PATCH] PR target/96759 - Handle global variable assignment from misaligned structure/PARALLEL return values.

[Kito Cheng <kito.cheng@sifive.com>]

Hi Richard:

Thanks for your review :)

> riscv doesn't seem to have movmisalign and I don't see why movmisalign
> should not support a TImode parallel RHS so at least you should
> put this check after the icode != CODE_FOR_nothing check?

RISC-V has an option `-mno-strict-align` to enable mis-aligned access,
but we didn't provide movmisalign pattern, I guess might be another issue,
I tried to add movmisalign pattern and then it will feed PARALLEL into that,
and got into a similar situation again, most targets are not except
got PARALLEL,
so that's the reason why I put before icode != CODE_FOR_nothing check.

> Also wouldn't it be better to support PARALLEL from within
> store_bit_field?
For the above reason, I think store_bit_field supports PARALLEL is  not enough.

> After all this is a misaligned access and
> since you didn't quote the RTL involved I'm guessing that
> emit_group_store knows nothing about this fact.

Oh, good point, how do you think about loading into temp and then
storing it in the DECL, so that we could use original logic to
handle misaligned store.

A PoC for this idea, didn't fully tested yet:

diff --git a/gcc/expr.c b/gcc/expr.c
index 1a15f24b3979..6ef97740c764 100644
--- a/gcc/expr.c
+++ b/gcc/expr.c
@@ -5166,8 +5166,19 @@ expand_assignment (tree to, tree from, bool nontemporal)
         || targetm.slow_unaligned_access (mode, align)))
    {
      rtx reg, mem;
+      push_temp_slots ();

      reg = expand_expr (from, NULL_RTX, VOIDmode, EXPAND_NORMAL);
+
+      if (GET_CODE (reg) == PARALLEL)
+       {
+         rtx temp = assign_stack_temp (mode,
+                                       GET_MODE_SIZE (mode));
+         emit_group_store (temp, reg, TREE_TYPE (from),
+                          int_size_in_bytes (TREE_TYPE (from)));
+         reg = temp;
+       }
+
      reg = force_not_mem (reg);
      mem = expand_expr (to, NULL_RTX, VOIDmode, EXPAND_WRITE);
      if (TREE_CODE (to) == MEM_REF && REF_REVERSE_STORAGE_ORDER (to))
@@ -5186,6 +5197,7 @@ expand_assignment (tree to, tree from, bool nontemporal)
      else
       store_bit_field (mem, GET_MODE_BITSIZE (mode), 0, 0, 0, mode, reg,
                        false);
+      pop_temp_slots ();
      return;
    }