Bounds checks: multiple monotonically increasing counters (within bounds)

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#116,238 3 comments 3 reactions 0 assignees View on GitHub

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Assessment

Difficulty
5/5
Estimated time
Over a week
Newbie friendliness
30/100
Issue type
Feature
Clarity
Needs clarification
Activity status
Stale
Tech stack
csharp
Domain
compilers, performance

Research direction

Start with the DecodeBytes_safe sample and compare its current generated assembly with the bounds-check behavior described in the issue. Investigate the JIT area responsible for optimizing multiple monotonically increasing counters; done means the generated loop no longer retains the unnecessary bounds check while preserving the required range-check failure behavior.

Written by the indexing model from the issue text.

Description

area-CodeGen-coreclr reduce-unsafe

A popular loop pattern when there are two (or more) monotonically increasing counters, unfortunately, JIT doesn't optimize bounds checks for them.

public int DecodeBytes_safe(Span<byte> buffer)
{
    int j = 0;
    for (int i = 0; i < buffer.Length; i++)
    {
        // e.g. under some condition
        buffer[j++] = 0;
    }
    return j;
}

Current codegen:

; Method Progam:DecodeBytes_safe(System.Span`1[ubyte]):int:this (FullOpts)
G_M20838_IG01:  ;; offset=0x0000
       sub      rsp, 40
G_M20838_IG02:  ;; offset=0x0004
       mov      rax, bword ptr [rdx]
       mov      ecx, dword ptr [rdx+0x08]
       xor      edx, edx
       test     ecx, ecx
       jg       SHORT G_M20838_IG06
G_M20838_IG03:  ;; offset=0x0010
       mov      r10d, edx
G_M20838_IG04:  ;; offset=0x0013
       mov      eax, r10d
G_M20838_IG05:  ;; offset=0x0016
       add      rsp, 40
       ret      
G_M20838_IG06:  ;; offset=0x001B
       mov      r8d, ecx
       align    [2 bytes for IG07]
G_M20838_IG07:  ;; offset=0x0020
       lea      r10d, [rdx+0x01]
       cmp      edx, ecx
       jae      SHORT G_M20838_IG09
       mov      edx, edx
       mov      byte  ptr [rax+rdx], 0
       dec      r8d
       je       SHORT G_M20838_IG04
G_M20838_IG08:  ;; offset=0x0033
       mov      edx, r10d
       jmp      SHORT G_M20838_IG07
G_M20838_IG09:  ;; offset=0x0038
       call     CORINFO_HELP_RNGCHKFAIL
       int3     
; Total bytes of code: 62
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