JIT: optimize (T)x | (T)y to (T)(x | y)

[EgorBo]

Noticed in https://github.com/dotnet/coreclr/issues/27917#issuecomment-557519701 We can optimize some movs away for patterns like (byte)x | (byte)y with (byte)(x | y), etc.
For example:

static int Downcast(int a, int b)
{
    return (byte)a | (byte)b;
}

static long Upcast(int a, int b)
{
    return (long)a & (long)b; // not only "|"
}

Current codegen:

; Method Foo:Downcast(int,int):int
       movzx    rax, cl
       movzx    rdx, dl
       or       eax, edx
       ret      


; Method Foo:Upcast(int,int):long
       movsxd   rax, ecx
       movsxd   rdx, edx
       and      rax, rdx
       ret      

Expected codegen (from godbolt and linux abi)

; Method Foo:Downcast(int,int):int
       or       edi, esi
       movzx    eax, dil
       ret


; Method Foo:Upcast(int,int):long
       and      edi, esi
       movsxd   rax, edi
       ret

Should be a simple optimization, e.g.:

\--*  OR        int   
   +--*  CAST      int <- ubyte <- int
   |  \--*  LCL_VAR   int    V00 arg0    
   \--*  CAST      int <- ubyte <- int
      \--*  LCL_VAR   int    V01 arg1   

To:

\--*  CAST      int <- ubyte <- int
   \--*  OR        int   
      +--*  LCL_VAR   int    V00 arg0    
      \--*  LCL_VAR   int    V01 arg1 

Also, should work for pointers:

static unsafe int DowncastPtr(int a, int b)
{
    return *((byte*)&a) | *((byte*)&b);
}

category:cq
theme:basic-cq
skill-level:beginner
cost:small

[vijaya-lakshmi-venkatraman]

Hi,
I would like to work on this issue if still available.
My understanding is that wherever we have the pattern as above, we can shorthand it.
Is that right?

[EgorBo]

Hi,
I would like to work on this issue if still available.
My understanding is that wherever we have the pattern as above, we can shorthand it.
Is that right?

Exactly, feel free to ask any questions

[vijaya-lakshmi-venkatraman]

Is there a specific folder(s) or set of files that I should consider for making these changes?

[trionia]

I was also wondering how to approach this issue and came up with the following shell command

(types="bool|s?byte|u?short|u?int|u?long"; find . -type f -name "*.cs" -exec grep -En "\(($types)\)\w+\s?(\||&)\s?\(\1\)\w+" {} \; -print)

I attached the result sample for the whole runtime directory. I'm not sure how and if the results should be narrowed down further.
runtime_13816.txt

[benjamin-hodgson]

I've opened a pull request for this: #58727