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Message-ID: <46B20CB2.7000103@gnu.org>
Date: Thu, 02 Aug 2007 16:56:00 -0000
From: Paolo Bonzini <bonzini@gnu.org>
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To: Andreas Krebbel <Andreas.Krebbel@de.ibm.com>
CC: gcc-patches@gcc.gnu.org
Subject: Re: [PATCH] lower-subreg: Decompose multiword shifts
References: <20070802153434.GA24953@blc4eb430604175.ibm.com>
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> Besides the fact that a shift decomposed into 2 simple move
> instructions can often be merged into other INSNs also the rest of
> lower subreg benefits since more multiword regs can be marked as
> decomposable.

Great! You could investigate changing the code so that the condition 
becomes

+   if (INTVAL (shift_count) < BITS_PER_WORD
+       || GET_MODE_BITSIZE (GET_MODE (shift_operand)) != 2 * BITS_PER_WORD)
+     return 0;

(your code has != in the first branch).  You can turn the code into a 
move+a shift, which simplifies to two moves if INTVAL (shift_count) == 
BITS_PER_WORD, in this case.

Paolo