[PATCH] D141560: [RISCV][CodeGen] Add codegen pattern for FLI instruction in experimental zfa extension

[Craig Topper via Phabricator via llvm-commits]

craig.topper added a comment.

In D141560#4212001 <https://reviews.llvm.org/D141560#4212001>, @joshua-arch1 wrote:

> In D141560#4198257 <https://reviews.llvm.org/D141560#4198257>, @craig.topper wrote:
>
>> In D141560#4198252 <https://reviews.llvm.org/D141560#4198252>, @joshua-arch1 wrote:
>>
>>> In D141560#4198233 <https://reviews.llvm.org/D141560#4198233>, @craig.topper wrote:
>>>
>>>> In D141560#4198220 <https://reviews.llvm.org/D141560#4198220>, @joshua-arch1 wrote:
>>>>
>>>>> Anyone knows how to generate FLI from C-code? If I compile the following program, I cannot get FlI. ConstantFP will be converted to Constant in DAG.
>>>>>
>>>>>   void foo_double64 ()
>>>>>   {
>>>>>     volatile double a;
>>>>>     a = 0.0625;
>>>>>   }
>>>>
>>>> You just need to use it to do some floating point arithmetic
>>>>
>>>>   void foo_double64 (double x)
>>>>   {
>>>>     volatile double a;
>>>>     a = x + 0.0625;
>>>>   }
>>>>
>>>> or return a floating point value
>>>>
>>>>   double foo_double64 ()
>>>>   {
>>>>     return 0.0625;
>>>>   }
>>>
>>> Is that because we cannot directly store a ConstantFP in DAG?
>>
>> The change is done in DAGCombiner::replaceStoreOfFPConstant. I think the idea is that FP constants are usually harder to create in registers than integer constants. Also CPUs  usually have more integer resources than FP resources. It doesn't look like it can be disabled currently. I think all of the constants FLI handles can be done in 1 or 2 integer instructions so I'm not very concerned about this. It was more than that I might be concerned.
>
> For rv64 in O0, replaceStoreOfFPConstant will not convert FP constant into integers. However, for rv32, FP constant will be changed to integers both in O0 and O2 <https://reviews.llvm.org/owners/package/2/>. Is there any special consideration?

I can see that it won't ever do it for float on rv64 because it doesn't consider the possibility of using an i32 store when i32 isn't a legal type.


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