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Date: Fri, 23 Mar 2007 08:40:00 -0000
From: "Chuan-Hua Chang" <chuanhua.chang@gmail.com>
To: cgen@sourceware.org
Subject: generated decoder code question
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In the utils-sim.scm file, the "-gen-decode-insn-entry" function has
the following lines:

		     ; Generate code to check that all of the opcode bits for this insn match
		     indent "    if (("
		     (if (adata-integral-insn? CURRENT-ARCH) "entire_insn" "base_insn")
		     " & 0x" (number->hex (insn-base-mask insn)) ") == 0x"
(number->hex (insn-value insn)) ")\n"
		     indent "      { itype = " (gen-cpu-insn-enum (current-cpu) insn) ";"
		     (if (with-scache?)
			 (if fn?
			     (string-append " @prefix@_extract_" fmt-name " (this,
current_cpu, pc, base_insn, entire_insn); goto done;")
			     (string-append " goto extract_" fmt-name ";"))
			 " goto done;")
		     " }\n"
		     indent "    " (-gen-decode-default-entry indent invalid-insn fn?)))))
)

It generates an IF-statement to check the opcode again inside the case
statement before doing the real extraction. This seems really
redundant and make the decoder inefficient.

I am wondering why this is needed here. Could someone help to explain
the reasoning?

When looking at the M32R decode function, the IF-statement is absent
from the decoder code.  This lets me wonder that if there is a way to
remove this redundant IF-statement check in the CGEN flow.

Thanks.