JIT: Strength reduction should be able to reason about field accesses on top of arrays

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#108,706 opened on Oct 9, 2024 1 comment 1 reaction 1 assignee View on GitHub

@jakobbotsch is already working on this.

Since Oct 9, 2024.

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This issue has not been assessed yet.

Description

area-CodeGen-coreclr Priority:3

From BenchmarksGame.Fasta:
https://github.com/dotnet/performance/blob/88a140a2234569c20129615ffea668ed857fc374/src/benchmarks/micro/runtime/BenchmarksGame/fasta-1.cs#L225-L233

Codegen on main:

...
G_M57319_IG03:  ;; offset=0x000B
       xor      edx, edx
       align    [3 bytes for IG04]
						;; size=5 bbWeight=0.25 PerfScore 0.12
G_M57319_IG04:  ;; offset=0x0010
       mov      r8, rdx
       vaddsd   xmm0, xmm0, qword ptr [rcx+r8+0x18]
       vmovsd   qword ptr [rcx+r8+0x18], xmm0
       add      rdx, 16
       dec      eax
       jne      SHORT G_M57319_IG04
						;; size=25 bbWeight=4 PerfScore 39.00
G_M57319_IG05:  ;; offset=0x0029
       ret      
						;; size=1 bbWeight=1 PerfScore 1.00

Codegen with #105267:

...
G_M57319_IG03:  ;; offset=0x000B
       add      rcx, 16
       align    [1 bytes for IG04]
						;; size=5 bbWeight=0.25 PerfScore 0.12
G_M57319_IG04:  ;; offset=0x0010
       vaddsd   xmm0, xmm0, qword ptr [rcx+0x08]
       vmovsd   qword ptr [rcx+0x08], xmm0
       add      rcx, 16
       dec      eax
       jne      SHORT G_M57319_IG04
						;; size=18 bbWeight=4 PerfScore 38.00
G_M57319_IG05:  ;; offset=0x0022
       ret      
						;; size=1 bbWeight=1 PerfScore 1.00

which is definitely an improvement, but we should be able to fold in the +0x08 offset that comes from the access of the p field. Currently we give up while trying to prove that rcx+0x08 is a valid managed byref in all iterations of the loop.

Dominant language: C#
Stars: 18.3k
Forks: 5.6k
PR merge metrics: PR metrics pending

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