> If we want to fix it in the combiner, I think the fix would be following.
> The optimization is about
> (and:SI (subreg:SI (reg:HI xxx) 0) (const_int 0x84c))
> and IMHO we can only optimize it into
> (subreg:SI (and:HI (reg:HI xxx) (const_int 0x84c)) 0)
> if we know that the upper bits of the REG are zeros.
The reasoning is that, for WORD_REGISTER_OPERATIONS, the subword AND operation
is done on the full word register, in other words that it's in effect:
(subreg:SI (and:SI (reg:SI xxx) (const_int 0x84c)) 0)
that is equivalent to the initial RTL so correct for WORD_REGISTER_OPERATIONS.
> Now, this patch fixes the PR, but certainly generates worse (but correct)
> code than the dse.cc patch. So perhaps we want both of them?
What happens if you disable the step I mentioned (patchlet attached)?
> As before, I unfortunately can't test it on riscv-linux (could perhaps try
> that on sparc-solaris on GCC Farm which is another WORD_REGISTER_OPERATIONS
> target, but last my bootstrap attempt there failed miserably because of the
> Don't bootstrap at midnight issue in cp/Make-lang.in; I'll post a patch
> for that once I test it).
I think that we have one in the Ada compiler too; glad to know we are not
alone in this boat. ;-)
--
Eric Botcazou
diff --git a/gcc/combine.cc b/gcc/combine.cc
index 053879500b7..5453ce85156 100644
--- a/gcc/combine.cc
+++ b/gcc/combine.cc
@@ -13321,14 +13321,12 @@ record_dead_and_set_regs_1 (rtx dest, const_rtx setter, void *data)
&& SUBREG_REG (SET_DEST (setter)) == dest
&& known_le (GET_MODE_PRECISION (GET_MODE (dest)),
BITS_PER_WORD)
- && subreg_lowpart_p (SET_DEST (setter)))
+ && subreg_lowpart_p (SET_DEST (setter))
+ && !(WORD_REGISTER_OPERATIONS
+ && paradoxical_subreg_p (SET_DEST (setter))
+ && contains_mem_rtx_p (SET_SRC (setter))))
record_value_for_reg (dest, record_dead_insn,
- WORD_REGISTER_OPERATIONS
- && word_register_operation_p (SET_SRC (setter))
- && paradoxical_subreg_p (SET_DEST (setter))
- ? SET_SRC (setter)
- : gen_lowpart (GET_MODE (dest),
- SET_SRC (setter)));
+ gen_lowpart (GET_MODE (dest), SET_SRC (setter)));
else
record_value_for_reg (dest, record_dead_insn, NULL_RTX);
}
