llvm dev - Jun 2019 - Using isConfused() method for LLVM Dependence Analysis

Hi,

I am trying to find out instances of confused dependencies in my 
program. According to LLVM's doxygen page, 'confused dependencies'
are
detected by function isConfused() - Returns true if this dependence is 
confused (the compiler understands nothing and makes worst-case 
assumptions) 
(http://llvm.org/doxygen/classllvm_1_1Dependence.html#a5c65e22ef4b0368ab040d1bd4ead382d)


For the following code snippet:

   for (int i = 0; i < 10; i++)
    {
        a = 1;
        for (int j = 0; j < 9; j++)
           {
              b = 2;
              for (int k = 0; k < 8; k++)
                 {
                      c = 3;
                 }
           }
    }
We are constructing dependencies between the corresponding IR of the 
above code and checking whether it is confused or not:

   store i32 1, i32* @a, align 4
   store i32 2, i32* @b, align 4
Confused Dependence or Not: 1

Since the variables 'a' and 'b' are in different memory
locations, why
is LLVM's isConfused method gives us True?
-- 
Bodhisatwa

Hello.

Are a, b and c globals in this example? How are you running this?

I believe depends() will return a pointer to a Dependence (technically a unique
pointer to a Dependence), which should be nullptr if there are no dependencies,
such as these cases that should be no-alias.

Confused dependencies are expected to come from cases where we don't know
the relationship between pointers. Such as this where we don't know the
relationship between a and b:
void test(int * a, int * b, int n)
{
  for (int i = 0; i < n; i++)
  {
    a[i] = 1;
    b[i] = 2;
  }
}

Dave



From: llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of
Bodhisatwa via llvm-dev <llvm-dev at lists.llvm.org>
Sent: 14 June 2019 17:50
To: llvm-dev at lists.llvm.org
Subject: [llvm-dev] Using isConfused() method for LLVM Dependence Analysis
 

Hi,

I am trying to find out instances of confused dependencies in my
program. According to LLVM's doxygen page, 'confused dependencies'
are
detected by function isConfused() - Returns true if this dependence is
confused (the compiler understands nothing and makes worst-case
assumptions)
(http://llvm.org/doxygen/classllvm_1_1Dependence.html#a5c65e22ef4b0368ab040d1bd4ead382d)


For the following code snippet:

   for (int i = 0; i < 10; i++)
    {
        a = 1;
        for (int j = 0; j < 9; j++)
           {
              b = 2;
              for (int k = 0; k < 8; k++)
                 {
                      c = 3;
                 }
           }
    }
We are constructing dependencies between the corresponding IR of the
above code and checking whether it is confused or not:

   store i32 1, i32* @a, align 4
   store i32 2, i32* @b, align 4
Confused Dependence or Not: 1

Since the variables 'a' and 'b' are in different memory
locations, why
is LLVM's isConfused method gives us True?
--
Bodhisatwa
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