This patch adds initial support for loop unrolling in the form of a
series of utility classes which perform the unrolling. The pass can
be run with the command spirv-opt --loop-unroll. This will unroll
loops within the module which have the unroll hint set. The unroller
imposes a number of requirements on the loops it can unroll. These are
documented in the comments for the LoopUtils::CanPerformUnroll method in
loop_utils.h. Some of the restrictions will be lifted in future patches.
This patch adds LoopUtils class to handle some loop related transformations. For now it has 2 transformations that simplifies other transformations such as loop unroll or unswitch:
- Dedicate exit blocks: this ensure that all exit basic block
(out-of-loop basic blocks that have a predecessor in the loop)
have all their predecessors in the loop;
- Loop Closed SSA (LCSSA): this ensure that all definitions in a loop are used inside the loop
or in a phi instruction in an exit basic block.
It also adds the following capabilities:
- Loop::IsLCSSA to test if the loop is in a LCSSA form
- Loop::GetOrCreatePreHeaderBlock that can build a loop preheader if required;
- New methods to allow on the fly updates of the loop descriptors.
- New methods to allow on the fly updates of the CFG analysis.
- Instruction::SetOperand to allow expression of the index relative to Instruction::NumOperands (to be compatible with the index returned by DefUseManager::ForEachUse)
