[Bug sim/31181] sim: cris: decode unused base_insn variable warnings

["vapier at gentoo dot org" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=31181

--- Comment #8 from Mike Frysinger <vapier at gentoo dot org> ---
(In reply to Hans-Peter Nilsson from comment #5)
> I don't think that's necessary: warnings are only enabled for gcc

they're enabled for toolchains that define __GNUC__.  llvm is known to do this
too.  i don't know the current status, but Intel's ICC has done this in the
past.  neither of those are gcc, and i don't like our build binding to CLI
behavior.

> Nah, that seems a bit overthinking it.  Adding it when warnings are enabled
> should be sufficient.  Alternatively, we can just add a "-Wno-error" like
> the extant cases.

-Wno-error makes it even easier for things to slip in.  there are plenty of
examples of this across software projects, including the sim.

> Developers can be expected to have a sufficiently modern gcc
> ...
> I don't have a new enough gcc to emit those particular ones

one person's modern is another's too-old ;)

> But, for that kind of change, this PR should be redirected to that project,
> as the template is there, in cgen/sim-decode.scm according to git grep.

sure, if we think the right place is cgen, i can send it there.  i wasn't sure
if it was something we'd do entirely there, or if we'd change cpu/cris.cpu, or
both.

ideally there would be something in the cpu definition that cgen could key of
off (like the arguments passed down, or what implicitly requested more fields
be decoded, or something else).  if that's not feasible, then i'd go with:

--- a/sim-decode.scm
+++ b/sim-decode.scm
@@ -436,7 +436,14 @@ void
    "    const IDESC *idesc = &" IDESC-TABLE-VAR "[itype];\n"
    (if (> (length (sfmt-iflds sfmt)) 0)
        (string-append
-        "    CGEN_INSN_WORD insn = "
+        "    "
+        ; Some insns are fully decoded by the time they get here, so they
won't
+        ; access the insn variable.  Mark it as unused to avoid warnings.
+        and don't need the i
+        (if (adata-integral-insn? CURRENT-ARCH)
+            ""
+            "ATTRIBUTE_UNUSED ")
+        "CGEN_INSN_WORD insn = "
         (if (adata-integral-insn? CURRENT-ARCH)
             "entire_insn;\n"
             "base_insn;\n"))

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