test.c

float test() {
    unsigned long a = 0xfffffffffffffffful;
    unsigned __int128 b = ((unsigned __int128)a << 64) | a;
    return (float)b;
}

test.ll

; ModuleID = 'test.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 readnone uwtable
define float @test() #0 {
  ret float undef
}

attributes #0 = { nounwind readnone uwtable "disable-tail-calls"="false" "less-precise-fpmad"="false" "no-frame-pointer-elim"="false" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+sse,+sse2" "unsafe-fp-math"="false" "use-soft-float"="false" }

!llvm.ident = !{!0}

!0 = !{!"clang version 3.7.1 (tags/RELEASE_371/final)"}