How Can IACA Help Optimize Instruction Scheduling for Intel Processors?

Understanding and Utilizing IACA

Introduction to IACA

Intel Architecture Code Analyzer (IACA) is a now-discontinued static analysis tool designed to optimize instruction scheduling on Intel processors. It analyzes compiled binaries with injected markers, allowing for insights into code execution patterns and resource utilization.

Injection of Markers

C/C :

#include "iacaMarks.h"

while (cond) {
    IACA_START
    // Loop body
    IACA_END
}

Assembly (x86):

    mov ebx, 111          ; Start marker bytes
    db 0x64, 0x67, 0x90   ; Start marker bytes

.innermostlooplabel:
    // Loop body
    jne .innermostlooplabel ; Conditional branch backwards to top of loop

    mov ebx, 222          ; End marker bytes
    db 0x64, 0x67, 0x90   ; End marker bytes

Analysis Execution

Run IACA with the following command:

iaca.sh -<bitness> -arch <architecture> -graph <output file> <binary>

Example:

iaca.sh -64 -arch HSW -graph insndeps.dot foo

Output Interpretation

IACA generates two types of output:

• Throughput Analysis Report:

  • Bottleneck identifications
  • Resource utilization in cycles per iteration

• Graphviz Dependency Graph:

  • Graphical representation of instruction dependencies

Example Analysis

Assembly Snippet:

.L2:
    vmovaps ymm1, [rdi+rax] ;L2
    vfmadd231ps ymm1, ymm2, [rsi+rax] ;L2
    vmovaps [rdx+rax], ymm1 ; S1
    add rax, 32 ; ADD
    jne .L2 ; JMP

Output (portion):

Intel(R) Architecture Code Analyzer Version - 2.1
...
Throughput Analysis Report
--------------------------
Block Throughput: 1.55 Cycles       Throughput Bottleneck: FrontEnd, PORT2_AGU, PORT3_AGU

The report identifies the bottleneck as the frontend and two AGUs on Haswell architecture.

Limitations

• Does not support certain instructions
• Limited to specific Intel processor generations
• Does not handle non-innermost loops in throughput mode (requires additional analysis tools such as LLVM-MCA)

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