cmd/compile: disallow rewrite rules from declaring reserved names

If I change a rule in ARM64.rules to use the variable name "b" in a
conflicting way, rulegen would previously not complain, and the compiler
would later give a confusing error:

	$ go run *.go && go build cmd/compile/internal/ssa
	# cmd/compile/internal/ssa
	../rewriteARM64.go:24236:10: b.NewValue0 undefined (type int64 has no field or method NewValue0)

Make rulegen complain early about those cases. Sometimes they might
happen to be harmless, but in general they can easily cause confusion or
unintended effect due to shadowing.

After the change, with the same conflicting rule:

	$ go run *.go && go build cmd/compile/internal/ssa
	2021/03/22 11:31:49 rule ARM64.rules:495 uses the reserved name b
	exit status 1

Note that 24 existing rules were using reserved names. It seems like the
shadowing was harmless, as it wasn't causing typechecking issues nor did
it seem to cause unintended behavior when the rule rewrite code ran.

The bool values "b" were renamed "t", since that seems to have a
precedent in other rules and in the fmt package.

Sequential values like "a b c" were renamed to "x y z", since "b" is
reserved.

Finally, "typ" was renamed to "_typ", since there doesn't seem to be an
obviously better answer.

Passes all three of:

	$ GOARCH=amd64 go build -toolexec 'toolstash -cmp' -a std
	$ GOARCH=arm64 go build -toolexec 'toolstash -cmp' -a std
	$ GOARCH=mips64 go build -toolexec 'toolstash -cmp' -a std

Fixes #45154.

Change-Id: I1cce194dc7b477886a9c218c17973e996bcedccf
Reviewed-on: https://go-review.googlesource.com/c/go/+/303549
Trust: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

1 files changed, 4 insertions, 4 deletions

diff --git a/src/cmd/compile/internal/ssa/rewritePPC64.go b/src/cmd/compile/internal/ssa/rewritePPC64.go
index a5bbc836cc..f181d433cc 100644
--- a/src/cmd/compile/internal/ssa/rewritePPC64.go
+++ b/src/cmd/compile/internal/ssa/rewritePPC64.go
@@ -1234,12 +1234,12 @@ func rewriteValuePPC64_OpConst8(v *Value) bool {
 	}
 }
 func rewriteValuePPC64_OpConstBool(v *Value) bool {
-	// match: (ConstBool [b])
-	// result: (MOVDconst [b2i(b)])
+	// match: (ConstBool [t])
+	// result: (MOVDconst [b2i(t)])
 	for {
-		b := auxIntToBool(v.AuxInt)
+		t := auxIntToBool(v.AuxInt)
 		v.reset(OpPPC64MOVDconst)
-		v.AuxInt = int64ToAuxInt(b2i(b))
+		v.AuxInt = int64ToAuxInt(b2i(t))
 		return true
 	}
 }