[llvm-dev] Dead Argument Elimination for Recursive Functions With Use in Recursion

[William Moses via llvm-dev]

Does dead argument elimination work on recursive functions where the value
of the recursive call is used to compute the next output?

I'm trying to have LLVM 7 eliminate the return value in the following code
snippet, where the return value of rdac (and the add) could be eliminated
but is not.

recurret.c
__attribute__((noinline))
static double rdac(double* x, unsigned n) {
    if (n == 1) {
        double val = x[0];
        x[0] = 0;
        return val;
    }
    return rdac(x, n/2) + rdac(x + n/2, n/2);
}

void dsum(double* x, unsigned n) {
    rdac(x, n);
}

recurret.ll:; ModuleID = 'recurret.c'
source_filename = "recurret.c"
target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"

; Function Attrs: nounwind uwtable
define dso_local void @dsum(double* %x, i32 %n) local_unnamed_addr #0 {
entry:
  %call = tail call fast fastcc double @rdac(double* %x, i32 %n)
  ret void
}

; Function Attrs: noinline nounwind uwtable
define internal fastcc double @rdac(double* %x, i32 %n) unnamed_addr #1 {
entry:
  %cmp = icmp eq i32 %n, 1
  br i1 %cmp, label %if.then, label %if.end

if.then:                                          ; preds = %entry
  %0 = load double, double* %x, align 8, !tbaa !2
  store double 0.000000e+00, double* %x, align 8, !tbaa !2
  ret double %0

if.end:                                           ; preds = %entry
  %div = lshr i32 %n, 1
  %call = tail call fast fastcc double @rdac(double* %x, i32 %div)
  %idx.ext = zext i32 %div to i64
  %add.ptr = getelementptr inbounds double, double* %x, i64 %idx.ext
  %call4 = tail call fast fastcc double @rdac(double* %add.ptr, i32 %div)
  %add = fadd fast double %call4, %call
  ret double %add
}

attributes #0 = { nounwind uwtable
"correctly-rounded-divide-sqrt-fp-math"="false"
"disable-tail-calls"="false" "less-precise-fpmad"="false"
"no-frame-pointer-elim"="false" "no-infs-fp-math"="true"
"no-jump-tables"="false" "no-nans-fp-math"="true"
"no-signed-zeros-fp-math"="true" "no-trapping-math"="true"
"stack-protector-buffer-size"="8" "target-cpu"="x86-64"
"target-features"="+fxsr,+mmx,+sse,+sse2,+x87" "unsafe-fp-math"="true"
"use-soft-float"="false" }
attributes #1 = { noinline nounwind uwtable
"correctly-rounded-divide-sqrt-fp-math"="false"
"disable-tail-calls"="false" "less-precise-fpmad"="false"
"no-frame-pointer-elim"="false" "no-infs-fp-math"="true"
"no-jump-tables"="false" "no-nans-fp-math"="true"
"no-signed-zeros-fp-math"="true" "no-trapping-math"="true"
"stack-protector-buffer-size"="8" "target-cpu"="x86-64"
"target-features"="+fxsr,+mmx,+sse,+sse2,+x87" "unsafe-fp-math"="true"
"use-soft-float"="false" }

!llvm.module.flags = !{!0}
!llvm.ident = !{!1}

!0 = !{i32 1, !"wchar_size", i32 4}
!1 = !{!"clang version 7.1.0 "}
!2 = !{!3, !3, i64 0}
!3 = !{!"double", !4, i64 0}
!4 = !{!"omnipotent char", !5, i64 0}
!5 = !{!"Simple C/C++ TBAA"}
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