[llvm-dev] LLVM IR intrinsics placeholder for strings [was Re: Back end with special loop instructions (using LLVM IR intrinsics)]

David Chisnall via llvm-dev
Tue, 14 Jun 2016 09:12:58 -0000

Hi,

Is there a reason that you can’t use inline assembly for this?

David

> On 13 Jun 2016, at 19:44, Alex Susu via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
>
>  Hello.
>    I come back to this thread. But I want to ask a slightly different
question.
>    Is there a way to have LLVM IR language intrinsics that are given at
construction time a string that is written at assembly generation time as it is?
(so, basically having placeholders of strings in LLVM that remain untouched
until the end, including code generation time.)
>
>    More exactly, I would like to give something LIKE this in a middle-tier
pass (I haven't tried this code in LLVM):
>     Value *instrinsicFunc = Intrinsic::getDeclaration(M,
>                                                Intrinsic::my_intrinsic);
>     myIRBuilder.CreateCall(repeatFunc,
>         "string_that_llc_codegen_should_output_as_assembly");
>   So my intrinsic would be accepting a general string which will be used at
codegen to be output. I guess I would have to define it like this in a .td file:
>    def int_my_intrinsic : Intrinsic<[], [string], []>;
>
>
>    The other option that can work but is more complex without any benefit
is to define several intrinsics in the .td files, one for EACH possible string:
>      // To put in the Intrinsics_....td file:
>      def int_my_intrinsic1 : Intrinsic<[], [], []>;
>      ...
>      def int_my_intrinsicN : Intrinsic<[], [], []>;
>      // To put in the InstrInfo.td file something like:
>      def STR_PLACEHOLDER1 : ImmediateInstruction< 0b111111,
>                                              (outs),
>                                              (ins),
>
"string_that_llc_codegen_should_output_as_assembly1",
>                                            [(int_my_intrinsic1)] >;
>      def STR_PLACEHOLDER2 : ImmediateInstruction< 0b111110,
>                                              (outs),
>                                              (ins),
>
"string_that_llc_codegen_should_output_as_assembly2",
>                                            [(int_my_intrinsic2)] >;
>
>
>  Best regards,
>    Alex
>
> On 6/5/2016 5:21 PM, Alex Susu wrote:
>>   Hello.
>>     Hal, the source file you mention
(lib/Target/PowerPC/PPCCTRLoops.cpp) makes use of
>> LLVM IR intrinsics, in this case defined at
>> [LLVM_repo]/llvm/include/llvm/IR/IntrinsicsPowerPC.td, such as:
>>           // Intrinsics used to generate ctr-based loops. These should
only be
>>       // generated by the PowerPC backend!
>>       def int_ppc_mtctr : Intrinsic<[], [llvm_anyint_ty], []>;
>>       def int_ppc_is_decremented_ctr_nonzero :
Intrinsic<[llvm_i1_ty], [], []>;
>>
>>     And these intrinsics are instantiated in the LLVM program, for
example with the
>> following code in PPCCTRLoops.cpp:
>>       IRBuilder<> CountBuilder(Preheader->getTerminator());
>>       Module *M = Preheader->getParent()->getParent();
>>       Value *MTCTRFunc = Intrinsic::getDeclaration(M,
Intrinsic::ppc_mtctr,CountType);
>>       CountBuilder.CreateCall(MTCTRFunc, ECValue);
>>
>>     I have defined also some intrinsics for my loop instructions in my
file
>> Intrinsics_Connex.td: 1 intrinsic for REPEAT_X_TIMES and 1 for
END_REPEAT.
>>       /* following Intrinsics.td:
>>       class Intrinsic<list<LLVMType> ret_types,
>>                 list<LLVMType> param_types = [],
>>                 list<IntrinsicProperty> properties = [],
>>                 string name = "">
>>       */
>>       def int_connex_repeat_x_times : Intrinsic<[], [], []>;
>>       def int_connex_end_repeat : Intrinsic<[llvm_i1_ty], [],
[]>;
>>    and added C++ code doing CreateCall() like the one above.
>>
>>     I'm looking now at http://llvm.org/docs/ExtendingLLVM.html on
how to specify the
>> instruction selection of this intrinsic. They write there:
>>       "Once the intrinsic has been added to the system, you must
add code generator
>> support for it. Generally you must do the following steps:
>>       Add support to the .td file for the target(s) of your choice in
lib/Target/*/*.td.
>>       This is usually a matter of adding a pattern to the .td file that
matches the
>> intrinsic, though it may obviously require adding the instructions you
want to generate as
>> well. There are lots of examples in the PowerPC and X86 backend to
follow."
>>
>>     Then in my pass I create a call to the respective intrinsic:
>>      /* See
http://llvm.org/docs/doxygen/html/classllvm_1_1TargetIntrinsicInfo.html :
>> "Create or insert an LLVM Function declaration for an intrinsic,
and return it."
>>       *   This creates a line in the LLVM program like: declare void
>> @llvm.connex.repeat.x.times() #2 .
>>       *     This line is required, otherwise llc will complain:
>>       *        <<error: use of undefined value
'@llvm.connex.repeat.x.times'
>>             call void @llvm.connex.repeat.x.times()>>
>>       */
>>       Value *repeatFunc = Intrinsic::getDeclaration(M,
>>
Intrinsic::connex_repeat_x_times);
>>
>>       // See
http://llvm.org/docs/doxygen/html/classllvm_1_1IRBuilder.html
>>       aB.CreateCall(repeatFunc); //, ECValue);
>>
>>
>>     Then, in the back end, in InstrInfo.td I write:
>>       let hasSideEffects = 1, isCodeGenOnly = 1 in {
>>       //let Pattern = [(int_connex_repeat_x_times)] in
>>       def REPEAT_X_TIMES : ImmediateInstruction< 0b111111,
>>                                             (outs),
>>                                             (ins),
>>
"REPEAT_X_TIMES(1001)",
>>
[(int_connex_repeat_x_times)] >;
>>       }
>>
>>
>>     Then, when I run opt and llc I obtain the expected behavior.
>>
>>   Thank you,
>>     Alex
>>
>>
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