[llvm-bugs] [Bug 41982] New: llvm-mca does not understand x86 zeroing idioms

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=41982

            Bug ID: 41982
           Summary: llvm-mca does not understand x86 zeroing idioms
           Product: tools
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: llvm-mca
          Assignee: unassignedbugs at nondot.org
          Reporter: brennan.vincent at gmail.com
                CC: andrea.dibiagio at gmail.com, llvm-bugs at lists.llvm.org,
                    matthew.davis at sony.com

This seems similar: https://bugs.llvm.org/show_bug.cgi?id=36671

It was marked fixed, but llvm-mca is still not recognizing xor as a zeroing
idiom in this simple test case. Note that the second `divl` has to wait for the
first to finish, despite the outputs being zeroed.


```
~ ❯❯❯ cat asm.s
        xorl %eax, %eax
        xorl %edx, %edx
        divl %edi
~ ❯❯❯ ~/clangmr/bin/llvm-mca -timeline -timeline-max-cycles=200  ~/asm.s       
                                                                               
                            ✘ 130 
<snip>


Timeline view:
                    0123456789          0123456789          0123456789         
0123456789          0123456789          0123456789          0123456789         
0123456789  
Index     0123456789          0123456789          0123456789         
0123456789          0123456789          0123456789          0123456789         
0123456789          01

[0,0]     DeER .    .    .    .    .    .    .    .    .    .    .    .    .   
.    .    .    .    .    .    .    .    .    .    .    .    .    .    .    .   
.    .    ..   xorl     %eax, %eax
[0,1]     DeER .    .    .    .    .    .    .    .    .    .    .    .    .   
.    .    .    .    .    .    .    .    .    .    .    .    .    .    .    .   
.    .    ..   xorl     %edx, %edx
[0,2]    
.DeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeER.
   .    .    .    .    .    .    .    .    .    .    .    .    .    .    .   
..   divl     %edi
[1,0]     .   
.D=======================================================================eER   
.    .    .    .    .    .    .    .    .    .    .    .    .    .    .    ..  
xorl     %eax, %eax
[1,1]     .   
.D=======================================================================eER   
.    .    .    .    .    .    .    .    .    .    .    .    .    .    .    ..  
xorl     %edx, %edx
[1,2]     .    .
D=======================================================================eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeER
  ..   divl     %edi
[2,0]     .    .    .
D==============================================================================================================================================eER
 ..   xorl     %eax, %eax
[2,1]     .    .    .
D==============================================================================================================================================eER
 ..   xorl     %edx, %edx
```

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