Rewriting the lower-buffer-element-type pass to avoid unnecessary packing/unpacking. (#8526)

Part of the effort to improve the performance of generated SPIRV code.

The existing lower-buffer-element-type pass works by loading the entire
buffer element content from memory, and translate it to logical type
stored in a local variable at the earliest reference of a buffer handle.
This means that is can generate inefficient code that reads more than
necessary.

Consider this example:
```
struct BigStruct { bool values[1024]; }
ConstantBuffer<BigStruct> cb;

void test(BigStruct v)
{
      if (v.values[0]) { printf("ok"); }
}

[numthreads(1,1,1)]
void computeMain()
{
    test(cb);
}
```

In IR, the `computeMain` function before lower-buffer-element-type pass
is something like following:
```
func test:
   %v = param : BigStruct
   %barr = fieldExtract(%v, "values")
   %element = elementExtract(%barr, 0)
    ... // uses %element 

func computeMain:
  %v = load(cb)
  call %test %v
```

The existing lower-buffer-element-type pass will rewrite the bool array
in `BigStruct` into `int` array so it is legal in SPIRV. However, it
does so by inserting the translation on the first `load` of the constant
buffer:

```
struct BigStruct_std430 {
    int values[1024];
}
var cb : ConstantBuffer<BigStruct_std430>;
func computeMain:
   %tmpVar : var<BigStruct>
    call %unpackStorage(%tmpVar, cb)
   %v : BigStruct = load %tmpVar
   call %test %v
```

This means that the entire array will be loaded and translated to int,
before calling `test`, which only uses one element. It turns out that
the downstream compiler isn't always able to optimize out this
inefficient translation/copy.

This PR completely rewrites the way buffer-element-type lowering is
handled to avoid producing this inefficient code. It works in two parts:
first we turn on the `transformParamsToConstRef` pass for SPIRV target
as well, so we will translate the `test` function to take the `v`
parameter as `constref`. The second part is a redesigned
buffer-element-type pass that defers the storage-type to logical-type
translation until a value is actually used by a `load` instruction.

In this example, after `transformParamsToConstRef`, the IR is:

```
func test:
   %v = param : ConstRef<BigStruct>
   %barr = fieldAddr(%v, "values")
   %elementPtr = elementAddr(%barr, 0)
   %element = load(%elementPtr)
    ... // uses %element 

func computeMain:
  call %test %cb
```

The new `buffer-element-type-lowering` pass will take this IR, and
insert translation at latest possible time across the entire call graph,
and translate the IR into:

```
func test:
   %v = param : ConstRef<BigStruct_std430>
   %barr = fieldAddr(%v, "values")
   %elementPtr : ptr<int> = elementAddr(%barr, 0)
   %element_int = load(%elementPtr)
    %element = cast(%element_int) : %bool
    ... // uses %element 

func computeMain:
  call %test %cb
```

In this new IR, there is no longer a load and conversion of the entire
array.

See new comment in `slang-ir-lower-buffer-element-type.cpp` for more
details of how the pass works.

This PR also address many other issues surfaced by turning on
`transformParamsToConstRef` pass on SPIRV backend.

---------

Co-authored-by: slangbot <186143334+slangbot@users.noreply.github.com>

1 files changed, 41 insertions, 28 deletions

diff --git a/source/slang/slang-ir.cpp b/source/slang/slang-ir.cpp
index e9d1a1199..92543c952 100644
--- a/source/slang/slang-ir.cpp
+++ b/source/slang/slang-ir.cpp
@@ -386,6 +386,20 @@ IRType* tryGetPointedToType(IRBuilder* builder, IRType* type)
     return nullptr;
 }
 
+IRType* tryGetPointedToOrBufferElementType(IRBuilder* builder, IRType* type)
+{
+    if (auto rateQualType = as<IRRateQualifiedType>(type))
+    {
+        type = rateQualType->getValueType();
+    }
+    auto resultType = tryGetPointedToType(builder, type);
+    if (resultType)
+        return resultType;
+    if (auto structuredBufferType = as<IRHLSLStructuredBufferTypeBase>(type))
+        return structuredBufferType->getElementType();
+    return nullptr;
+}
+
 
 // IRBlock
 
@@ -5480,39 +5494,19 @@ IRInst* IRBuilder::emitElementAddress(IRInst* basePtr, IRInst* index)
     }
     IRType* type = nullptr;
     valueType = unwrapAttributedType(valueType);
-    if (auto arrayType = as<IRArrayTypeBase>(valueType))
-    {
-        type = arrayType->getElementType();
-    }
-    else if (auto vectorType = as<IRVectorType>(valueType))
-    {
-        type = vectorType->getElementType();
-    }
-    else if (auto coopVecType = as<IRCoopVectorType>(valueType))
-    {
-        type = coopVecType->getElementType();
-    }
-    else if (auto matrixType = as<IRMatrixType>(valueType))
-    {
-        type = getVectorType(matrixType->getElementType(), matrixType->getColumnCount());
-    }
-    else if (auto coopMatType = as<IRCoopMatrixType>(valueType))
-    {
-        type = coopMatType->getElementType();
-    }
-    else if (const auto basicType = as<IRBasicType>(valueType))
+    if (as<IRBasicType>(valueType))
     {
         // HLSL support things like float.x, in which case we just return the base pointer.
         return basePtr;
     }
-    else if (const auto tupleType = as<IRTupleType>(valueType))
-    {
-        SLANG_ASSERT(as<IRIntLit>(index));
-        type = (IRType*)tupleType->getOperand(getIntVal(index));
-    }
-    else if (auto hlslInputPatchType = as<IRHLSLInputPatchType>(valueType))
+    type = getElementType(*this, (IRType*)valueType);
+    if (!type)
     {
-        type = hlslInputPatchType->getElementType();
+        if (const auto tupleType = as<IRTupleType>(valueType))
+        {
+            SLANG_ASSERT(as<IRIntLit>(index));
+            type = (IRType*)tupleType->getOperand(getIntVal(index));
+        }
     }
 
     SLANG_RELEASE_ASSERT(type);
@@ -6179,6 +6173,24 @@ IRInst* IRBuilder::emitCastIntToPtr(IRType* ptrType, IRInst* val)
     return inst;
 }
 
+IRInst* IRBuilder::emitCastStorageToLogical(IRType* type, IRInst* val, IRInst* bufferType)
+{
+    if (type == val->getDataType())
+        return val;
+    IRInst* args[] = {val, bufferType};
+    return (IRCastStorageToLogical*)emitIntrinsicInst(type, kIROp_CastStorageToLogical, 2, args);
+}
+
+IRCastStorageToLogicalDeref* IRBuilder::emitCastStorageToLogicalDeref(
+    IRType* type,
+    IRInst* val,
+    IRInst* bufferType)
+{
+    IRInst* args[] = {val, bufferType};
+    return (IRCastStorageToLogicalDeref*)
+        emitIntrinsicInst(type, kIROp_CastStorageToLogicalDeref, 2, args);
+}
+
 IRGlobalConstant* IRBuilder::emitGlobalConstant(IRType* type)
 {
     auto inst = createInst<IRGlobalConstant>(this, kIROp_GlobalConstant, type);
@@ -6627,6 +6639,7 @@ IRInst* IRBuilder::emitGenericAsm(UnownedStringSlice asmText)
 IRInst* IRBuilder::emitRWStructuredBufferGetElementPtr(IRInst* structuredBuffer, IRInst* index)
 {
     const auto sbt = cast<IRHLSLStructuredBufferTypeBase>(structuredBuffer->getDataType());
+    SLANG_ASSERT(sbt);
     const auto t = getPtrType(sbt->getElementType());
     IRInst* const operands[2] = {structuredBuffer, index};
     const auto i = createInst<IRRWStructuredBufferGetElementPtr>(