https://github.com/shader-slang/slang
Tip revision: cf7dddae74ae990c2b46a9feeaea3b7a33ec077d authored by Theresa Foley on 02 September 2021, 23:10:53 UTC
Two small fixes. (#1928)
Two small fixes. (#1928)
Tip revision: cf7ddda
slang-ir-restructure-scoping.cpp
// slang-ir-restructure-scoping.cpp
#include "slang-ir-restructure-scoping.h"
#include "slang-ir.h"
#include "slang-ir-insts.h"
#include "slang-ir-restructure.h"
namespace Slang
{
/// Try to find the first structured region that represents `block`
///
/// In general the same block may appear as multiple regions,
/// so this will return the first region in the linked list.
static SimpleRegion* getFirstRegionForBlock(
RegionTree* regionTree,
IRBlock* block)
{
SimpleRegion* region = nullptr;
if( regionTree->mapBlockToRegion.TryGetValue(block, region) )
{
return region;
}
return nullptr;
}
/// Try to find the first structured region that contains `inst`.
static SimpleRegion* getFirstRegionForInst(
RegionTree* regionTree,
IRInst* inst)
{
auto ii = inst;
while(ii)
{
if(auto block = as<IRBlock>(ii))
return getFirstRegionForBlock(regionTree, block);
ii = ii->getParent();
}
return nullptr;
}
/// Compute the depth of a node in the region tree.
///
/// This is the number of nodes (including `region`)
/// on a path from `region` to the root.
///
static Int computeDepth(Region* region)
{
Int depth = 0;
for( Region* rr = region; rr; rr = rr->getParent() )
{
depth++;
}
return depth;
}
/// Get the `n`th ancestor of `region`.
///
/// When `n` is zero, this returns `region`.
/// When `n` is one, this returns the parent of `region`, and so forth.
///
static Region* getAncestor(Region* region, Int n)
{
Region* rr = region;
for( Int ii = 0; ii < n; ++ii )
{
SLANG_ASSERT(rr);
rr = rr->getParent();
}
return rr;
}
/// Find a region that is an ancestor of both `left` and `right`.
static Region* findCommonAncestorRegion(
Region* left,
Region* right)
{
// Rather than blinding search through each ancestor of `left`
// and see if it is also an ancestor of `right` and vice-versa,
// let's try to be smart about this.
//
// We will start by computing the depth of `left` and `right`:
//
Int leftDepth = computeDepth(left);
Int rightDepth = computeDepth(right);
// Whatever the common ancestor is, it can't be any deeper
// than the minimum of these two depths.
//
Int minDepth = Math::Min(leftDepth, rightDepth);
// Let's fetch the ancestor of each of `left` and `right`
// corresponding to that depth:
//
Region* leftAncestor = getAncestor(left, leftDepth - minDepth);
Region* rightAncestor = getAncestor(right, rightDepth - minDepth);
// Now we know that `leftAncestor` and `rightAncestor`
// must have the same depth. Let's go ahead and assert
// it just to be safe:
//
SLANG_ASSERT(computeDepth(leftAncestor) == minDepth);
SLANG_ASSERT(computeDepth(rightAncestor) == minDepth);
// If `leftAncestor` and `rightAncestor` are the same node,
// then we've found a common ancestor, otherwise we should
// look at their parents. Because the depth must match
// on both sides, we will never risk missing an ancestor.
//
while( leftAncestor != rightAncestor )
{
leftAncestor = leftAncestor->getParent();
rightAncestor = rightAncestor->getParent();
}
// Okay, we've found a common ancestor.
//
Region* commonAncestor = leftAncestor;
return commonAncestor;
}
/// Find a simple region that is an ancestor of both `left` and `right`.
static SimpleRegion* findSimpleCommonAncestorRegion(
Region* left,
Region* right)
{
// Start by finding a common ancestor without worrying about it being simple.
Region* ancestor = findCommonAncestorRegion(left, right);
// Now search for a simple region up the tree.
while( ancestor )
{
if(ancestor->getFlavor() == Region::Flavor::Simple)
return (SimpleRegion*) ancestor;
ancestor = ancestor->getParent();
}
// This shouldn't ever occur. The root of the region tree should
// be a simple regions that represents the entry block of the
// function.
//
SLANG_UNEXPECTED("no common ancestor found in region tree");
UNREACHABLE_RETURN(nullptr);
}
IRInst* getDefaultInitVal(
IRBuilder* builder,
IRType* type)
{
switch( type->getOp() )
{
default:
return nullptr;
case kIROp_BoolType:
return builder->getBoolValue(false);
case kIROp_IntType:
case kIROp_UIntType:
case kIROp_UInt64Type:
return builder->getIntValue(type, 0);
case kIROp_HalfType:
case kIROp_FloatType:
case kIROp_DoubleType:
return builder->getFloatValue(type, 0.0);
// TODO: handle vector/matrix types here, by
// creating an appropriate scalar value and
// then "splatting" it.
}
}
/// Initialize a variable to a sane default value, if possible.
void defaultInitializeVar(
IRBuilder* builder,
IRVar* var,
IRType* type)
{
IRInst* initVal = nullptr;
switch( type->getOp() )
{
case kIROp_VoidType:
default:
// By default, see if we can synthesize an IR value
// to be used as the default, and allow the logic
// below to store it into the variable.
initVal = getDefaultInitVal(builder, type);
break;
// TODO: Handle aggregate types (structures, arrays)
// explicitly here, since they need to be careful about
// the cases where an element/field type might not
// be something we can default-initialize.
}
if( initVal )
{
builder->emitStore(var, initVal);
}
}
/// Detect and fix any structured scoping issues for a given `def` instruction.
///
/// The `defRegion` should be the region that contains `def`, and `regionTree`
/// should be the region tree for the function that contains `def`.
///
static void fixValueScopingForInst(
IRInst* def,
SimpleRegion* defRegion,
RegionTree* regionTree)
{
// This algorithm should not consider "phi nodes" for now,
// because the emit logic will already create variables for them.
// We could consider folding the logic to move out of SSA form
// into this function, but that would add a lot of complexity for now.
if(def->getOp() == kIROp_Param)
return;
// We would have a scoping violation if there exists some
// use `u` of `def` such that the region containing `u`
// (call it `useRegion`) is not a descendent of `defRegion`
// in the region tree.
//
// If there are no scoping violations, we don't want to do
// anything. If there *are* any scoping violations, then
// we ill need to introduce a temporary `tmp`, store into
// it right after `def`, and then load from it at any "bad"
// use sites.
//
// Of course, for the whole thing to work, we also need
// to put `tmp` into a block somwhere, and it needs to
// be a block that is visible to all of the uses, or we
// are just back int the same mess again.
//
// The right block to use for inserting `tmp` is the least
// common ancestor of `def` and all the "bad" uses, so
// we will get a bit "clever" and fold in the search for
// bad uses with the computation of the region we should
// insert `tmp` into (to avoid looping over the uses
// twice).
//
SimpleRegion* insertRegion = defRegion;
IRVar* tmp = nullptr;
// If we end up needing to insert code we'll need an IR builder,
// so we will go ahead and create one now.
//
// TODO: the logic to compute `module` here could be hoisted
// out earlier, rather than being done per-instruction.
//
IRModule* module = regionTree->irCode->getModule();
SharedIRBuilder sharedBuilder;
sharedBuilder.session = module->session;
sharedBuilder.module = module;
IRBuilder builder;
builder.sharedBuilder = &sharedBuilder;
// Because we will be changing some of the uses of `def`
// to use other values while we iterate the list, we
// need to be a bit careful and extract the next use
// in the linked list *before* we operator on `u`.
//
IRUse* nextUse = nullptr;
for( auto u = def->firstUse; u; u = nextUse )
{
nextUse = u->nextUse;
// Looking at the use site `u`, we'd like to check if
// it violates our scoping rules.
//
// As a simple early-exit case, if the user is in
// the same block as the definition, there are no problems.
//
IRInst* user = u->getUser();
if(user->getParent() == defRegion->block)
continue;
// Otherwise, let's find the structures control-flow
// region that holds the user. We expect to always
// find one, because the use site must be in the same
// function.
//
// TODO: Double check that logic if we ever introduce
// things like nested function.
//
SimpleRegion* useRegion = getFirstRegionForInst(regionTree, user);
// If there is no region associated with the use, then
// the use must be in unreachable code (part of the CFG,
// but not part of the region tree). We will skip
// such uses for now, since they won't even appear in
// the output.
//
if(!useRegion)
continue;
// Now we want to check if `useRegion` is a child/descendent
// of a region that has the same block as `defRegion`.
// If it is, then there is no scoping problem with this use.
//
if(useRegion->isDescendentOf(defRegion->block))
continue;
// If we've gotten this far, we know that `u` is a "bad"
// use of `def`, and needs fixing.
//
// We will create the `tmp` variable on demand, so
// that we create it when the first "bad" use is encountered,
// and then re-use it for subsequent bad uses.
//
if( !tmp )
{
// We will create a temporary to represent `def`,
// and insert a `store` into it right after `def`.
//
// Note: we are inserting the new variable right
// after `def` for now, just because we don't
// yet know the final region that it should be
// placed into. We will move it to the correct
// location when we are done.
//
builder.setInsertBefore(def->getNextInst());
tmp = builder.emitVar(def->getDataType());
builder.emitStore(tmp, def);
}
// In order to know where `tmp` should be defined
// at the end of the algorithm, we need to compute
// a valid `insertRegion` that is an ancestor of
// all of the use sites (and it also a simple region
// so that we can insert into its IR block).
//
// We need to deal with one complexity in our restructuring
// process, which is that a block may be duplicated into
// one or more regions, so we loop over all the regions
// for the same block as `useRegion`.
//
for(auto rr = useRegion; rr; rr = rr->nextSimpleRegionForSameBlock)
{
insertRegion = findSimpleCommonAncestorRegion(
insertRegion,
rr);
}
// To fix up the use `u`, we will need to change
// it from using `def` to using a load from `tmp`
//
builder.setInsertBefore(user);
IRInst* tmpVal = builder.emitLoad(tmp);
// We are clobbering the value used by the `IRUse` `u`,
// while will cut it out of the list of uses for `def`.
// We need to be careful when doing this to not disrupt
// our iteration of the uses of `def`, so we carefully
// used the `nextUse` temporary at the start of the loop.
//
u->set(tmpVal);
}
// At the end of the loop, the `tmp` variable will have
// been created if and only if we fixed up anything.
//
if( tmp )
{
// If we created a temporary, then now we need to move
// its definition to the right place, which is the
// `insertRegion` that we computed during the loop.
//
// We'd like to insert our temporary near the top
// of the region, since that is the conventional
// place for local variables to go.
//
tmp->insertBefore(
insertRegion->block->getFirstOrdinaryInst());
// The whole point of the transformation we are doing
// here is that `def` is not on the "obvious" control
// flow path to one or more uses (which are now using
// `tmp`), but that means that it might not be "obvious"
// to a downstream compiler that `tmp` always gets
// initialized (by the code we inserted after `def`)
// before each of these use sites.
//
// We *know* that things are valid as long as our
// dominator tree was valid - there is no way to
// get to the block that loads from `tmp` without passing
// through the block that computes `def` (and then
// stores it into `tmp`) first.
//
// To avoid warnings/errros, we will go ahead and try
// to emit logic to "default initialize" the `tmp`
// variable if possible.
//
builder.setInsertBefore(tmp->getNextInst());
defaultInitializeVar(&builder, tmp, def->getDataType());
}
}
void fixValueScoping(RegionTree* regionTree)
{
// We are going to have to walk through every instruction
// in the code of the function to detect an bad cases.
//
auto code = regionTree->irCode;
for(auto block : code->getBlocks())
{
// All of the instruction in `block` will have the same
// parent region, so we will look it up now rather than
// have to re-do this work on a per-instruction basis.
//
auto parentRegion = getFirstRegionForBlock(regionTree, block);
// If a block has no region then it must be unreachable,
// so we will skip it entirely for this pass.
//
// TODO: we should be eliminating unrechable blocks anyway.
//
if(!parentRegion)
continue;
for(auto inst : block->getDecorationsAndChildren())
{
fixValueScopingForInst(inst, parentRegion, regionTree);
}
}
}
}