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Date: Sat, 13 Mar 2004 09:24:00 -0000
From: Zdenek Dvorak <rakdver@atrey.karlin.mff.cuni.cz>
To: Devang Patel <dpatel@apple.com>
Cc: "gcc@gcc.gnu.org list" <gcc@gcc.gnu.org>,
	Jason Merrill <jason@redhat.com>, Dorit Naishlos <DORIT@il.ibm.com>,
	Richard Henderson <rth@redhat.com>,
	Diego Novillo <dnovillo@redhat.com>
Subject: Re: [lno] [RFC] if-conversion and auto vectorizer
Message-ID: <20040313092438.GA6767@atrey.karlin.mff.cuni.cz>
References: <415253BF-7455-11D8-A347-000393A91CAA@apple.com>
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Hello,

> OK. So far I have coded transformation to transform
> 
> bar ()
> {
>   int A[N+1], B[N+1], C[N+1], D[N+1];
>   int i;
> 
>   for (i = 1; i<N; i++)
>     {
>       if (A[i] > 0)
>         {
>           A[i] = A[i] + C[i];
>           B[i] = A[i] + 1;
>         }
>       else
>         {
>           A[i] = D[i];
>           B[i] = D[i] + 1;
>         }
>     }
>   ibar(A);
>   ibar(B);
> }
> 
> into :
> 
> bar ()
> {
>   int i;
>   int D[17];
>   int C[17];
>   int B[17];
>   int A[17];
>   int T.5;
>   int T.4;
>   int T.3;
>   int T.2;
>   int T.1;
>   int T.0;
> 
>   # BLOCK 0
>   # PRED: ENTRY [100.0%]  (fallthru,exec)
>   # SUCC: 1 [100.0%]  (fallthru,exec)
> 
>   # BLOCK 1
>   # PRED: 6 [100.0%]  (fallthru) 0 [100.0%]  (fallthru,exec)
>   # i_1 = PHI <1(0), i_15(6)>;
> <L0>:;
>   T.0_3 = if (1)
>     {
>       A[i_1];
>     };
>   if (T.0_3 > 0) goto <L1>; else goto <L2>;
>   # SUCC: 3 [21.0%]  (false,exec) 2 [79.0%]  (true,exec)
> 
>   # BLOCK 2
>   # PRED: 1 [79.0%]  (true,exec)
> <L1>:;
>   T.1_9 = if (1 && T.0_3 > 0)

why this "1 &&" part?

>     {
>       C[i_1];
>     };
>   T.2_10 = if (1 && T.0_3 > 0)
>     {
>       T.0_3 + T.1_9;
>     };
>   A[i_1] = if (1 && T.0_3 > 0)
>     {
>       T.2_10
>     };
>   T.3_12 = if (1 && T.0_3 > 0)
>     {
>       T.2_10 + 1;
>     };
>   B[i_1] = if (1 && T.0_3 > 0)
>     {
>       T.3_12
>     };
>   goto <bb 4> (<L3>);
>   # SUCC: 4 [100.0%]  (fallthru,exec)
> 
>   # BLOCK 3
>   # PRED: 1 [21.0%]  (false,exec)
> <L2>:;
>   T.4_5 = if (1 && !(T.0_3 > 0))
>     {
>       D[i_1];
>     };
>   A[i_1] = if (1 && !(T.0_3 > 0))
>     {
>       T.4_5
>     };
>   T.5_7 = if (1 && !(T.0_3 > 0))
>     {
>       T.4_5 + 1;
>     };
>   B[i_1] = if (1 && !(T.0_3 > 0))
>     {
>       T.5_7
>     };
>   # SUCC: 4 [100.0%]  (fallthru,exec)
> 
>   # BLOCK 4
>   # PRED: 3 [100.0%]  (fallthru,exec) 2 [100.0%]  (fallthru,exec)
> <L3>:;
>   i_15 = i_1 + 1;
>   if (i_15 <= 15) goto <L8>; else goto <L5>;
>   # SUCC: 5 [11.0%]  (false,exec) 6 [89.0%]  (true,exec)
> 
>   # BLOCK 6
>   # PRED: 4 [89.0%]  (true,exec)
> <L8>:;
>   goto <bb 1> (<L0>);
>   # SUCC: 1 [100.0%]  (fallthru)
> 
>   # BLOCK 5
>   # PRED: 4 [11.0%]  (false,exec)
> <L5>:;
>   ibar (&A);
>   ibar (&B);
>   return;
>   # SUCC: EXIT [100.0%]
> 
> }
> 
> I spent lot of time unnecessary to create super block inside loop by
> merging block 2 and 3 into block 1.
> 
> If vectorizer can not vectorize this loop then it will be reverted to
> original form using loop versioning. Am I on the right track ?

I am not at all sure what you want to achieve with this transformation;
it seems to me that you haven't done anything that could possibly make
vectorizer's work easier.

Zdenek