[llvm-dev] EFLAGS

[Chris Sears via llvm-dev]

What is the difference between

    Uses = [EFLAGS] + Defs = [EFLAGS]

and their mention in selection patterns?

    [(set GR64:$dst, EFLAGS, (X86smul_flag GR64:$src1, GR64:$src2))]

Basically, my problem is that I have weird instructions which undef the EFLAGS register, leaving EFLAGS in an unknown state. For example:

    cmp x, y
    beq z           ; this is good because EFLAGS is defined

    cmp x, y
    weird_inst.   ; undefs EFLAGS, leaves EFLAGS in a undefined state
    beq z           ; this is bad because EFLAGS is undefined

Is it enough to just say Defs = [EFLAGS] for the Instruction def of weird_inst? Or is it necessary for the pattern to say something like [(set EFLAGS, undef)] as well?

[Craig Topper via llvm-dev]

There can only be one "set" operator per pattern list. I think we usually do "(implicit EFLAGS)" after the set, but I don't think it is required. I think it allows tablegen to infer Defs = [EFLAGS] from the (implicit EFLAGS), but I don't know of any place that doesn't just list the Defs explicitly.

~Craig

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[Chris Sears via llvm-dev]

thanks, implicit does make sense now. Resources for understanding it are:

1. inferring from examples in other backends (AArch64, AVR, MSP430 and X86)
2. TableGen source
3. not the MIR docs

But it's not hard.

C