dotnet/runtime

Could emit branchless form of i <= 0b00111 && j <= 0b00111 for unsigned integers

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#92,692 opened on Sep 27, 2023

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Description

Similar to #61940, equivalent to (i | j) <= 0b0011, the constant value can be 2^n-1, also the condition can be <.

For example:

bool Test0(ulong x, ulong y) => x <= UInt32.MaxValue && y <= UInt32.MaxValue;

bool Test1(ulong x, ulong y) => (x | y) <= UInt32.MaxValue;

The JIT could produce the same output for the above, but currently not, see https://godbolt.org/z/G9Taxqcox

C:Test(ulong,ulong):ubyte:this (FullOpts):
       mov      eax, 0xD1FFAB1E
       cmp      rsi, rax
       ja       SHORT G_M42689_IG05
       mov      eax, 0xD1FFAB1E
       cmp      rdx, rax
       setbe    al
       movzx    rax, al
       ret      
G_M42689_IG05:
       xor      eax, eax
       ret      

C:Test1(ulong,ulong):ubyte:this (FullOpts):
       or       rdx, rsi
       mov      eax, 0xD1FFAB1E
       cmp      rdx, rax
       setbe    al
       movzx    rax, al
       ret

Contributor guide

Research direction
Investigate the JIT's lowering phase for boolean conditions to see how to handle range checks like `x <= constant && y <= constant` and transform them into `(x | y) <= constant` when constant is 2^n 1.
Tech stack
csharp
Domain
backend
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Prerequisites
C#JIT compilation
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40