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llvm cookbook 1.7 优化ir

llvm cookbook 1.7 优化ir

作者: peteyuan | 来源:发表于2018-11-09 17:27 被阅读21次

    本文介绍如何使用opt工具优化llvm ir。

    使用之前编写的代码 multiply.c

    int mult() {
      int a = 5;
      int b = 3;
      int c = a * b;
      return c;
    }
    

    执行命令

    clang -emit-llvm -S multiply.c -o multiply.ll
    

    生成 multiply.ll

    ; ModuleID = 'multiply.c'
    source_filename = "multiply.c"
    target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
    target triple = "x86_64-apple-macosx10.14.0"
    
    ; Function Attrs: noinline nounwind optnone ssp uwtable
    define i32 @mult() #0 {
    entry:
      %a = alloca i32, align 4
      %b = alloca i32, align 4
      %c = alloca i32, align 4
      store i32 5, i32* %a, align 4
      store i32 3, i32* %b, align 4
      %0 = load i32, i32* %a, align 4
      %1 = load i32, i32* %b, align 4
      %mul = mul nsw i32 %0, %1
      store i32 %mul, i32* %c, align 4
      %2 = load i32, i32* %c, align 4
      ret i32 %2
    }
    
    attributes #0 = { noinline nounwind optnone ssp uwtable "correctly-rounded-divide-sqrt-fp-math"="false" "disable-tail-calls"="false" "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-jump-tables"="false" "no-nans-fp-math"="false" "no-signed-zeros-fp-math"="false" "no-trapping-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="penryn" "target-features"="+cx16,+fxsr,+mmx,+sse,+sse2,+sse3,+sse4.1,+ssse3,+x87" "unsafe-fp-math"="false" "use-soft-float"="false" }
    
    !llvm.module.flags = !{!0, !1}
    !llvm.ident = !{!2}
    
    !0 = !{i32 1, !"wchar_size", i32 4}
    !1 = !{i32 7, !"PIC Level", i32 2}
    !2 = !{!"clang version 7.0.0 (trunk 324834)"}
    

    执行命令:

    opt -mem2reg -S multiply.ll -o multiply1.ll
    

    输出 multiply1.ll

    ; ModuleID = 'multiply.ll'
    source_filename = "multiply.c"
    target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
    target triple = "x86_64-apple-macosx10.14.0"
    
    ; Function Attrs: noinline nounwind optnone ssp uwtable
    define i32 @mult() #0 {
    entry:
      %a = alloca i32, align 4
      %b = alloca i32, align 4
      %c = alloca i32, align 4
      store i32 5, i32* %a, align 4
      store i32 3, i32* %b, align 4
      %0 = load i32, i32* %a, align 4
      %1 = load i32, i32* %b, align 4
      %mul = mul nsw i32 %0, %1
      store i32 %mul, i32* %c, align 4
      %2 = load i32, i32* %c, align 4
      ret i32 %2
    }
    
    attributes #0 = { noinline nounwind optnone ssp uwtable "correctly-rounded-divide-sqrt-fp-math"="false" "disable-tail-calls"="false" "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-jump-tables"="false" "no-nans-fp-math"="false" "no-signed-zeros-fp-math"="false" "no-trapping-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="penryn" "target-features"="+cx16,+fxsr,+mmx,+sse,+sse2,+sse3,+sse4.1,+ssse3,+x87" "unsafe-fp-math"="false" "use-soft-float"="false" }
    
    !llvm.module.flags = !{!0, !1}
    !llvm.ident = !{!2}
    
    !0 = !{i32 1, !"wchar_size", i32 4}
    !1 = !{i32 7, !"PIC Level", i32 2}
    !2 = !{!"clang version 7.0.0 (trunk 324834)"}
    

    我在mac上执行的,不知道为什么结果和书上的不一致,这个优化好像并没有生效。

    其他的优化选项还有

    • adce: Aggressive Dead Code Elimination
    • bb-vectorize: Basic-Block Vectorization
    • constprop: Simple constant propagation
    • dce: Dead Code Elimination
    • deadargelim: Dead Argument Elimination
    • globaldce: Dead Global Elimination
    • globalopt: Global Variable Optimizer
    • gvn: Global Value Numbering
    • inline: Function Integration/Inlining
    • instcombine: Combine redundant instructions
    • licm: Loop Invariant Code Motion
    • loop: unswitch: Unswithch Loop
    • loweratomic: Lower atomic intrinsics to non-atomic form
    • lowerinvoke: Lower invokes to calls, for unwindless code generators
    • lowerswitch: Lower SwithcInsts to branches
    • mem2reg: Promote Memory to Registry
    • memcpyopt: MemCpy Optimization
    • simplifycfg: Simplify the CFG
    • sink: Code sinking
    • tailcallelim: Tail Call Elimination

    可以在源码目录 test/Transforms/ 下找到测试代码。

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