cmd/compile: rulegen results in confusing compiler errors when reserved names are used in rewrite rules

[zhangfannie]

Recently, I have a problem when I want to rewrite the following go codes with ARM64 REV16 instruction.

// go codes

 func genREV16(c uint64) uint64 {
        b := ((c & 0xff00ff00) >> 8) | ((c & 0x00ff00ff) << 8)
        return b
 }

The above codes can be optimized as two instructions "AND 0xffffffff, c, Rd" and "REV16 Rd". So I added the following rewritten rules to do this optimization.

((ADDshiftLL|ORshiftLL|XORshiftLL) <typ.UInt64> [8] (SRLconst [8] (ANDconst [a] x)) (ANDconst [b] x))
        && (uint64(a) == 0xff00ff00 && uint64(b) == 0x00ff00ff)
        => (REV16 (ANDconst <typ.UInt64> [0xffffffff] x))

But make.bash will report the error: "golong/src/cmd/compile/internal/ssa/rewriteARM64.go:1845: b.NewValue0 undefined (type int64 has no field or method NewValue0)".

[zhangfannie]

Thanks @josharian. Josh figured out the reason for the failure, which is the variable "b“ (and "v" and some others) is reserved in rewritten rules.

How can we improve the detection of reserved names? Thank you.

[gopherbot]

Change https://golang.org/cl/303549 mentions this issue: cmd/compile: disallow rewrite rules from declaring reserved names