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, 12 insertions, 2 deletions

diff --git a/source/slang/slang-ir-undo-param-copy.cpp b/source/slang/slang-ir-undo-param-copy.cpp
index d8aac7201..75ee8a03a 100644
--- a/source/slang/slang-ir-undo-param-copy.cpp
+++ b/source/slang/slang-ir-undo-param-copy.cpp
@@ -6,7 +6,7 @@
 
 namespace Slang
 {
-// This pass transforms variables decorated with TempCallArgVarDecoration
+// This pass transforms variables decorated with TempCallArgImmutableVarDecoration
 // by replacing them with direct references to the original parameters.
 // This is important for CUDA/OptiX targets where functions like 'IgnoreHit'
 // can prevent copy-back operations from executing.
@@ -52,7 +52,17 @@ struct UndoParameterCopyVisitor
             {
                 if (auto varInst = as<IRVar>(inst))
                 {
-                    if (varInst->findDecoration<IRTempCallArgVarDecoration>())
+                    bool isTempCallArgVar = false;
+                    for (auto decor : varInst->getDecorations())
+                    {
+                        if (as<IRTempCallArgImmutableVarDecoration>(decor) ||
+                            as<IRTempCallArgVarDecoration>(decor))
+                        {
+                            isTempCallArgVar = true;
+                            break;
+                        }
+                    }
+                    if (isTempCallArgVar)
                     {
                         IRStore* initializingStore = nullptr;
                         IRInst* originalParamPtr = nullptr;