[LLVMdev] Intrinsics __readeflags and __writeeflags

[David Majnemer]

This intrinsic seems very ill-defined, apparently it can be freely
reordered and does _not_ act like a compiler barrier. [1]
Other than source compatibility, why would one want this intrinsic?  What
semantics is it supposed to give?

[1] <
http://connect.microsoft.com/VisualStudio/feedback/details/691456/-readeflags-intrinsic-can-be-reordered-by-the-compiler
>


On Tue, Dec 17, 2013 at 11:00 AM, Reid Kleckner <rnk at google.com> wrote:

> I don't know enough about LLVM CodeGen to answer your questions.  I'm just
> curious.
>
> What is the intended level of support for these intrinsics?  Are they for
> reading ALU flags like CF, OF, etc, or for seldom changed control flags
> like TF and AC?  Even DF is typically scratch, and could be used for an -Oz
> memmove lowering for example.
>
> I don't think LLVM will ever really support capturing ALU flags from
> previous ops without "using" the operation.  LLVM does have overflow
> intrinsics though:
> http://llvm.org/docs/LangRef.html#id1164
>
>
> On Tue, Dec 17, 2013 at 1:02 AM, Alexey Volkov <avolkov.intel at gmail.com>wrote:
>
>> Hello all,
>>
>> I am trying to implement intrinsics __readeflags and __writeeflags
>> reading and writing EFLAGS register on x86.
>> These intrinsics expand to two instructions popf and push to register for
>> __readeflags and pushf and pop to register for __writeeflags.
>> These instructions are not connected explicitly so I can't use patterns
>> in .td file to match intrinsics.
>>
>> I tried to implement custom expansion making COPY DAG node with copy from
>> EFLAGS to register.
>> But this solution works only at -O0 level and failed at -O1 and higher:
>> the problem is that Post-RA pseudo instruction expansion pass seems to be
>> called only at -O0.
>>
>> Another way is to expand intrinsics to DAG nodes for each PUSH, POP,
>> PUSHF and POPF instructions.
>> This will add 4 new X86ISD types for DAG nodes for these instructions.
>>
>> What is the proper way to expand these intrinsics?
>>
>> --
>> Alexey Volkov
>> Intel Corporation
>>
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