[LLVMdev] phi instuction example

[Eli Bendersky]

On Fri, Apr 18, 2014 at 10:09 AM, <lionheart8470 at gmail.com> wrote:

> Hello. I'm a newbie for LLVM.
>
> What I'm trying to do is to write the simplest program containing phi IR.
> However, even though I wrote and if statements, what I get is LLVM IR not
> containing phi node. I tried the examples shown in LLVM documentation but
> it didn't work.
>
> At first, I thought it is because of the optimization level but without
> optimization, it doesn't produce phi, and when using optimization level
> higher and equal to O1, the meaningless while loops are just removed, so I
> have a problem.
>
> I think it is easy problem but, for some reasons that I don't know, I
> failed making one.
>
> Can any give me simplest program producing phi?
>

An easy way to get a phi is to run the mem2reg pass with opt. For example,
this C code:

int foo(int a, int b) {
  if (a > b) return a + b + 2;
  else return a * b * 17;
}




When compiled with Clang to LLVM IR, and run through 'opt -mem2reg',
produces:




; ModuleID = 'f.opt.bc'
target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"

; Function Attrs: nounwind
define i32 @foo(i32 %a, i32 %b) #0 {
entry:
  %cmp = icmp sgt i32 %a, %b
  br i1 %cmp, label %if.then, label %if.else

if.then:                                          ; preds = %entry
  %add = add nsw i32 %a, %b
  %add1 = add nsw i32 %add, 2
  br label %return

if.else:                                          ; preds = %entry
  %mul = mul nsw i32 %a, %b
  %mul2 = mul nsw i32 %mul, 17
  br label %return

return:                                           ; preds = %if.else,
%if.then
  %retval.0 = phi i32 [ %add1, %if.then ], [ %mul2, %if.else ]
  ret i32 %retval.0
}

attributes #0 = { nounwind "less-precise-fpmad"="false"
"no-frame-pointer-elim"="false" "no-infs-fp-math"="false"
"no-nans-fp-math"="false" "no-realign-stack"
"stack-protector-buffer-size"="8" "unsafe-fp-math"="false"
"use-soft-float"="false" }

!llvm.ident = !{!0}

!0 = metadata !{metadata !"clang version 3.5.0 "}


Eli
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