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>

2 files changed, 59 insertions, 0 deletions

diff --git a/tests/optimization/arrray-storage-lowering.slang b/tests/optimization/arrray-storage-lowering.slang
new file mode 100644
index 000000000..42bb8f127
--- /dev/null
+++ b/tests/optimization/arrray-storage-lowering.slang
@@ -0,0 +1,42 @@
+// TEST:SIMPLE(filecheck=SPV): -target spirv
+
+// TEST(compute, vulkan):COMPARE_COMPUTE_EX(filecheck-buffer=CHECK):-vk -compute -shaderobj -output-using-type -emit-spirv-directly
+
+struct DoubleNested
+{
+    int4x3 matrix;
+    int getMatVal(int i, int j) { return matrix[i][j]; }
+}
+
+struct Nested
+{
+    bool values[4];
+    DoubleNested doubleNested;
+    int getVal(int id) { return (int)values[0] + doubleNested.getMatVal(0, 1); }
+}
+
+struct Params
+{
+    Nested nested;
+
+    int getVal(int id) { return nested.getVal(id) + nested.getVal(id + 1); }
+}
+
+// TEST_INPUT: set outputBuffer = out ubuffer(data=[0], stride=4)
+RWStructuredBuffer<int4> outputBuffer;
+
+// TEST_INPUT:set gParams = cbuffer(data=[1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1])
+ConstantBuffer<Params> gParams;
+
+// TEST_INPUT: set gDoubleNested = ubuffer(data=[1 2 3 4 5 6 7 8 9 10 11 12])
+uniform DoubleNested *gDoubleNested;
+
+// CHECK: 9
+
+[numthreads(1,1,1)]
+void computeMain(int id: SV_DispatchThreadID)
+{
+    outputBuffer[0].xyz = gParams.getVal(id) + gDoubleNested.getMatVal(1, 1);
+}
+
+// SPV-NOT: OpCompositeConstruct
diff --git a/tests/optimization/get-array-element.slang b/tests/optimization/get-array-element.slang
new file mode 100644
index 000000000..16a71aee2
--- /dev/null
+++ b/tests/optimization/get-array-element.slang
@@ -0,0 +1,17 @@
+//TEST:SIMPLE(filecheck=CHECK):-target spirv
+
+int test(int arr[32]) {
+    int sum = 0;
+    for (int i =0; i < 32; i++) sum += arr[i];
+    return sum;
+}
+
+uniform int gArr[32];
+uniform int* result;
+
+[numthreads(1,1,1)]
+void computeMain()
+{
+    // CHECK-NOT: OpCompositeConstruct
+    *result = test(gArr);
+}
\ No newline at end of file