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Simplify arithmetic operations on registry and memory#125559

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pedrobsaila wants to merge 8 commits intodotnet:mainfrom
pedrobsaila:125300
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Simplify arithmetic operations on registry and memory#125559
pedrobsaila wants to merge 8 commits intodotnet:mainfrom
pedrobsaila:125300

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@pedrobsaila pedrobsaila commented Mar 14, 2026 •
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Fixes #125300

Lacks tests, working on it

Before :

Program:AddReg(int):int (FullOpts):
       lea      eax, [rdi+2*rdi]
       ret      

Program:AddMem(byref):int (FullOpts):
       mov      eax, dword ptr [rdi]
       lea      ecx, [rax+rax]
       add      eax, ecx
       ret      

Program:SubReg(int):int (FullOpts):
       xor      eax, eax
       sub      eax, edi
       ret      

Program:SubMem(byref):int (FullOpts):
       mov      eax, dword ptr [rdi]
       mov      ecx, eax
       sub      ecx, eax
       sub      ecx, eax
       mov      eax, ecx
       ret

After:

Program:AddReg(int):int (FullOpts):
       lea      eax, [rdi+2*rdi]
       ret      

Program:AddMem(byref):int (FullOpts):
       mov      eax, dword ptr [rdi]
       lea      eax, [rax+2*rax]
       ret      

Program:SubReg(int):int (FullOpts):
       mov      eax, ecx
       neg      eax
       ret      

Program:SubMem(byref):int (FullOpts):
       mov      eax, dword ptr [rdi]
       neg      eax
       ret

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EgorBo commented Mar 14, 2026 •
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haven't checked the correctness of these changes, but the diffs imply that +300 LOC to JIT are not worth it.

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pedrobsaila commented Mar 14, 2026 •
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There is room to reduce LOC, I just wrote it fast to see whether their will be asm diffs or not. I'll roll more compact version in a few hours. But I agree the diffs are not that impressive.

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EgorBo commented Mar 14, 2026 •
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There is room to reduce LOC, I just wrote it fast to see whether their will be asm diffs or not. I'll roll more compact version in a few hours. But I agree the diffs are not that impressive.

Right. Generally, we don't accept PRs like that unless there is a strong justification. We see it as an extra risk that is not worth the impact. To be honest, all kinds of x + x + x .. transformation should be handled elsewhere, on VN/SSA level.

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pedrobsaila commented Mar 14, 2026

To be honest, all kinds of x + x + x .. transformation should be handled elsewhere, on VN/SSA level.

Sorry to bother you, but never worked on VN/SSA level, I need to ask you about which file the transformation should be valuenum.cpp/ssabuilder.cpp or elsewhere ?

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EgorBo commented Mar 14, 2026

To be honest, all kinds of x + x + x .. transformation should be handled elsewhere, on VN/SSA level.

Sorry to bother you, but never worked on VN/SSA level, I need to ask you about which file the transformation should be valuenum.cpp/ssabuilder.cpp or elsewhere ?

I just tried myself with VN and it didn't find diffs either (mostly because we don't materialize VNs today). With SSA it's probably a more advanced task. probably like in copyprop or assertionprop, but it might also not worth it - hard to tell in advance.

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JIT: Simplify arithmetic operations on memory, as is already done for register

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