[LLVMdev] Opt pass 'Canonicalize Induction Variables' not working

[Tobias Grosser]

On 01/16/2012 01:19 PM, Pieter Custers wrote:
> I am new to LLVM and I've a question about the optimization passes and
> in particular about the loop transformation passes. I've setup the LLVM
> tool-chain with Polly installed.
>
> The following example is causing problems with me:
> http://llvm.org/docs/Passes.html#indvars
>
> The input is a .c file with the following loops:
>
> # 01 Standard, N=100
> for (i=0; i<N; i++)
> b[i] = a[i] + 5;
>
> # 02 Example loop -indvars
> for (i = 7; i*i < 1000; ++i)
> b[i] = 4;
>
> # 03 Start at 1 instead of 0
> for (i = 1; i<10; i++)
> b[i] = 3;
>
> I have the latest LLVM build installed on Mac OSX 10.6.8.
>
> A script with (among others) the following lines is used:
>
> -----------------------------------------------------------------------------------------------------
> POLLY_DLIB=/Developer/llvm_build/tools/polly/Debug+Asserts/lib/LLVMPolly.dylib
> alias opt="/Developer/llvm_build/Debug+Asserts/bin/opt -load ${POLLY_DLIB}"
> alias llvm-as="/Developer/llvm_build/Debug+Asserts/bin/llvm-as"
>
> # Set input filename
> FILE="indvar"
>
> # Run clang to generate LLVM-IR
> clang -S -emit-llvm ${FILE}.c -o ${FILE}.s
>
> # Make the human readable format into bitcode format
> llvm-as ${FILE}.s -o ${FILE}.bc
>
> # Prepare the LLVM-Bytecode into canonical form
> opt -S -mem2reg -loop-simplify -indvars ${FILE}.bc > ${FILE}.ll
> -----------------------------------------------------------------------------------------------------
>
>
> What I want (and what is not working) is that loop 2 and loop 3 are
> transformed into a loop starting at 0 with stride 1 (natural loop).
> Subsequent passes need this to output information to form a Polyhedral
> model (PHM).
>
> Up to now, only the first loop is recognized as natural loop (as it
> already is one) and information about the loop is outputted as PHM. The
> other two loops aren't changed.
>
> Can somebody give me some advise? Maybe I'm using the wrong passes or in
> the wrong order?

Hi Pieter,

the -indvars pass does by default not generate canonical induction 
variables. Most LLVM passes where converted to work without canonical 
induction variables. For Polly those induction variables are (still) 
required. You can enable the relevant conversion by adding
-enable-iv-rewrite to the opt flags.

In general, the to code for Polly a larger set of canonicalization 
passes is required. They can be run from clang with

clang -Xclang -load ${POLLY_DLIB} ${FILE}.c -o ${FILE}.ll -S -emit-llvm 
-mllvm -polly -O0

You need to make sure that Polly, LLVM and clang are compiled from the 
same source version.

Cheers
Tobi