From: "Owen Anderson" <firstname.lastname@example.org>
To: "Jakob Stoklund Olesen" <email@example.com>
Cc: "LLVM Developers Mailing List" <firstname.lastname@example.org>
Sent: Tuesday, January 7, 2014 1:09:05 PM
Subject: Re: [LLVMdev] Random question about the x86 backend (and backends in general I suppose)
From: "Craig Topper" < email@example.com >
To: "Chandler Carruth" < firstname.lastname@example.org >
Cc: "LLVM Developers Mailing List" < email@example.com >
Sent: Monday, December 30, 2013 2:29:50 PM
Subject: Re: [LLVMdev] Random question about the x86 backend (and
backends in general I suppose)
I can't speak directly to the questions themselves, but I'll ask a
couple back. When you say that some instructions are missing
mayLoad, do these instructions have patterns? Tablegen can infer
mayLoad/mayStores/hasSideEffects from patterns so it doesn't always
need to be listed explicitly in the td files.
Having recently audited these flags in the PowerPC backend, I highly
recommend looking at these from the *GenInstrInfo.inc file directly.
I find this much easier. In theory, we'd like to move away from the
pattern-based flag inference. Once a target is free of dependence on
the inference rules, it can set bit guessInstructionProperties = 0;
to turn them off completely (see class InstrInfo in Target.td).
In MHO, we should try to avoid redundancy as much as possible. The
only reason to have these flags is when instructions don't have
I'm fairly certain that Jakob put in an error (warning?) when
tablegen detects redundant flags.
Tablegen will emit an error if the specified flags are inconsistent
with the patterns.
If you clear the guessInstructionProperties flag, it will also refuse
to infer flags from patterns, while still verifying the manually
specified flags against the patterns when possible.
I’d like to throw out that having the inference present can be a
massive problem at time. It’s reasonably common to have a multi
class that is reused for a number of different instructions, say
different types of load. Some of the instantiations of the class
have a pattern (say a normal i32 load) and some don’t (load of an
illegal type, for example). In this case, you need to set the isLoad
flag on the defs in the multiclass in order the make sure the latter
get it properly, but you’ll get warnings (errors?) from tblgen about
redundant flags on the former. The typical workaround is to
duplicate the multiclass for with-flags and without-flags, which is
even more useless duplication than having the flags ever was.
I'd like to second this. The problem with the current inference scheme is that it is not consistent: instructions sometimes get inferred flags, depending on how their patterns are specified, and sometimes don't. Furthermore, auditing those inferred flags is not easy, and because this is a correctness issue, they do need to be checked.
My preference would be for TableGen to warn if a flag that is implied by a pattern is missing from the instruction, and nothing more than that.