Re: [PATCH] combine: Fix simplify_comparison AND handling for WORD_REGISTER_OPERATIONS targets [PR109040]

[Eric Botcazou <botcazou@adacore.com>]

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> 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

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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);
     }