[LLVMdev] Converting into SSA form

[Chayan Sarkar]

Hi Jeffrey,

Actually I am trying to implement "E-path PRE" which is based on
non-algebric equivallence. So, the  variable names need to be
preserved.

You said that I need to insert these to preserve variable
%a.0 = bitcast i32 2 to i32

So, these need to be inserted before the mem2reg pass or within the pass.
In first case, how to call an inbuilt pass after doing some analysis
from my own pass?
In second case, how to change/modify an inbuilt pass?

Please elaborate on this.

Thank you,
Chayan

On Sun, Jun 6, 2010 at 1:32 AM, Jeffrey Yasskin <jyasskin at google.com> wrote:
> There is no existing pass to do this in LLVM, mostly because it
> wouldn't be useful for optimizing programs. From your input, mem2reg
> produces:
>
> define i32 @myfunc(i32 %x, i32 %y) nounwind {
> entry:
>  %cmp = icmp sgt i32 %x, %y                      ; <i1> [#uses=1]
>  br i1 %cmp, label %if.then, label %if.else
>
> if.then:                                          ; preds = %entry
>  %add = add nsw i32 2, 3                         ; <i32> [#uses=1]
>  br label %if.end
>
> if.else:                                          ; preds = %entry
>  %mul = mul i32 2, 3                             ; <i32> [#uses=1]
>  br label %if.end
>
> if.end:                                           ; preds = %if.else, %if.then
>  %a.0 = phi i32 [ 6, %if.then ], [ 2, %if.else ] ; <i32> [#uses=1]
>  %b.0 = phi i32 [ 3, %if.then ], [ 4, %if.else ] ; <i32> [#uses=1]
>  %c.0 = phi i32 [ %add, %if.then ], [ %mul, %if.else ] ; <i32> [#uses=1]
>  %add8 = add nsw i32 %c.0, %a.0                  ; <i32> [#uses=1]
>  %add11 = add nsw i32 %add8, %b.0                ; <i32> [#uses=1]
>  ret i32 %add11
> }
>
> In order to preserve the constants' names from the original program,
> mem2reg would have to insert operations like
>
> %a.0 = bitcast i32 2 to i32
>
> which subsequent optimizers would have to remove again, slowing down
> compilation unnecessarily.
>
> The original program does have an instruction
>  store i32 %add11, i32* %c
> Without inserting extraneous instructions, mem2reg could rename %add11
> to %c.1 when it eliminates that store, which might make the optimized
> form a little easier to read. On the other hand, doing that would slow
> down mem2reg (I don't know how much), so it shouldn't be on by
> default.
>
> What are you really trying to do?
>
> On Sat, Jun 5, 2010 at 5:53 AM, Chayan Sarkar <chayan.ju at gmail.com> wrote:
>> Suppose my Input function is like :
>>
>> myfunc(int x,int y){
>>    int a=2, b=3,c=5;
>>    if(x>y) {
>>        c=a+b;
>>        a=6;
>>    }
>>    else {
>>        c=a*b;
>>        b=4;
>>    }
>>    a=c+a;
>>    c=a+b;
>> }
>>
>> and the output should be :
>>
>> myfunc(int x,int y){
>>    int a.0=2, b.0=3,c.0=5;
>>    if(x>y) {
>>        c.1=a.0+b.0;
>>        a.1=6;
>>    }
>>    else {
>>        c.2=a.0*b.0;
>>        b.1=4;
>>    }
>>    a.2=phi(a.1,a.0);
>>    b.2=phi(b.0,b.1);
>>    c.3=phi(c.1,c.2);
>>    a.3=c.3+a.2;
>>    c.4=a.3+b.2;
>> }
>>
>>
>> Thank you for your response.
>>
>>
>> On Sat, Jun 5, 2010 at 3:03 PM, Eli Friedman <eli.friedman at gmail.com> wrote:
>>> On Sat, Jun 5, 2010 at 2:03 AM, Chayan Sarkar <chayan.ju at gmail.com> wrote:
>>>> But, the mem2reg pass removes all load store instructions. It replaces
>>>> all variables by their if possible (kind of constant propagation). I
>>>> have generated the bitcode of the source program and the applied the
>>>> mem2reg pass and obviously not getting desired thing.
>>>>
>>>> What I want is convert it into SSA form without replacing any variable
>>>> by their constant value. Please elaborate on your point.
>>>
>>> Can you give a short sample program and what you expect the output to look like?
>>>
>>> -Eli
>>>
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