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Date: Wed, 16 Jan 2013 11:47:14 +0100
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH 2/3] optimize: track nonzero bits of
	registers
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To: Jay Foad <jay.foad@gmail.com>
Cc: qemu-devel@nongnu.org, Richard Henderson <rth@twiddle.net>

Il 16/01/2013 10:09, Jay Foad ha scritto:
>> @@ -621,6 +627,87 @@ static TCGArg *tcg_constant_folding(TCGContext *s,
>> uint16_t *tcg_opc_ptr,
>>              break;
>>          }
>>
>> +        /* Simplify using known-zero bits */
>> +        mask = -1;
>> +        switch (op) {
>> +        CASE_OP_32_64(ext8s):
>> +            if ((temps[args[1]].mask & 0x80) != 0) {
>> +                break;
>> +            }
>> +        CASE_OP_32_64(ext8u):
>> +            mask = 0xff;
>> +            goto and_const;
>> +        CASE_OP_32_64(ext16s):
>> +            if ((temps[args[1]].mask & 0x8000) != 0) {
>> +                break;
>> +            }
>> +        CASE_OP_32_64(ext16u):
>> +            mask = 0xffff;
>> +            goto and_const;
>> +        case INDEX_op_ext32s_i64:
>> +            if ((temps[args[1]].mask & 0x80000000) != 0) {
>> +                break;
>> +            }
>> +        case INDEX_op_ext32u_i64:
>> +            mask = 0xffffffffU;
>> +            goto and_const;
>> +
>> +        CASE_OP_32_64(and):
>> +            mask = temps[args[2]].mask;
>> +            if (temps[args[2]].state == TCG_TEMP_CONST) {
>> +        and_const:
>> +                ;
>> +            }
> 
> What's the purpose of this "if"?

It is filled in by patch 3/3.

Paolo