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Consider the following simple function:

int foo() { return 42; }

Compiling this to LLVM via clang -emit-llvm -S foo.cpp produces the following module:

; ModuleID = 'foo.cpp'
source_filename = "foo.cpp"
target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-apple-macosx10.13.0"

; Function Attrs: noinline nounwind ssp uwtable
define i32 @_Z3foov() #0 {
  ret i32 42
}

attributes #0 = { noinline nounwind 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}
!llvm.ident = !{!1}

!0 = !{i32 1, !"PIC Level", i32 2}
!1 = !{!"Apple LLVM version 9.0.0 (clang-900.0.37)"}

Why is the foo function declared as noinline? The flag is not added if an optimization level (other than -O0) is specified, but I would like to avoid that.

Is there another way / flag?

jfrohnhofen
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  • Makes sense to me that, for an unoptimized build (that needs to be debugable), functions are not inlined. Why do you want it otherwise? – Jesper Juhl Oct 17 '17 at 16:16
  • I want to modify / analyze it further with an external tool (and hence need it to be unoptimized), but want to be able to pass the modified module to `opt` later and have it optimized then... – jfrohnhofen Oct 17 '17 at 16:23

1 Answers1

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With -O0, you can't enable inlining globally, judging from Clang's source code (Frontend\CompilerInvocation.cpp):

// At O0 we want to fully disable inlining outside of cases marked with
// 'alwaysinline' that are required for correctness.
Opts.setInlining((Opts.OptimizationLevel == 0)
                  ? CodeGenOptions::OnlyAlwaysInlining
                  : CodeGenOptions::NormalInlining);

Depending on your requirements, you may:

  • Use -O1, which is the closest to -O0.
  • Use -O1 in conjuction with disabling of optimization flags that it enables. See the following answer for optimization flags enabled with -O1: Clang optimization levels
  • Apply always_inline attribute selectively on functions that should be inlined.
    For example: int __attribute__((always_inline)) foo() { return 42; }
valiano
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