llvm dev - Jan 2019 - minimal C code that would generate IR code using phi

Hi,

I am trying to understand phi better. I know what it means.

https://en.wikipedia.org/wiki/Static_single_assignment_form

But when I tried to create a minimal code using if-else with
unbalanced statements to change a variable in C, it still did not
generate IR code using phi.

Does anybody have a minimal C code that would generate IR code using
phi? Thanks.

-- 
Regards,
Peng

On Fri, 25 Jan 2019 at 16:55, Peng Yu via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> Does anybody have a minimal C code that would generate IR code using
> phi? Thanks.
Clang never really generates phis itself because it's easier to
generate code that simply uses a stack variable (you don't have to
keep track of control flow). The Mem2Reg pass in LLVM is what converts
Clang's output into a form with phis.

If you run Clang at an optimization above -O0 (e.g. "clang -O1 -Xclang
-disable-llvm-passes -emit-llvm ...") then run "opt -mem2reg"
manually
you'll closest to what Clang would have emitted if it did do phis.

Cheers.

Tim.

On 1/25/2019 10:55 AM, Peng Yu via llvm-dev wrote:
> 
> Does anybody have a minimal C code that would generate IR code using
> phi? Thanks.
I don't know if this is minimal, but it's short:

--- p.c ---
int bar(int a);

int foo(int a) {
   return a > 0 ? bar(a-1) : -bar(0);
}
-----------

$ clang -S -emit-llvm -O2 p.c -o -

; ModuleID = 'p.c'
source_filename = "p.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 i32 @foo(i32 %a) local_unnamed_addr #0 {
entry:
   %cmp = icmp sgt i32 %a, 0
   br i1 %cmp, label %cond.true, label %cond.false

cond.true:                                        ; preds = %entry
   %sub = add nsw i32 %a, -1
   %call = tail call i32 @bar(i32 %sub) #2
   br label %cond.end

cond.false:                                       ; preds = %entry
   %call1 = tail call i32 @bar(i32 0) #2
   %sub2 = sub nsw i32 0, %call1
   br label %cond.end

cond.end:                                         ; preds = %cond.false, 
%cond.true
   %cond = phi i32 [ %call, %cond.true ], [ %sub2, %cond.false ]
   ret i32 %cond
}
[...]


-Krzysztof