Enhance buffer load specialization pass to specialize past field extracts. (#8547)

This allows us to specialize functions whose argument is a sub element
of a constant buffer, instead of being only applicable to entire buffer
element. Closes #8421.

This change also implements a proper heuristic to determine when to
specialize the calls and defer the buffer loads.

This PR addresses a pathological case exposed in
`slangpy\slangpy\benchmarks\test_benchmark_tensor.py`, which used to
take 27ms to finish, and now takes 1.25ms.


For example, given:
```
struct Bottom
{
    float bigArray[1024];

    [mutating]
    void setVal(int index, float value) { bigArray[index] = value; }
}

struct Root
{
    Bottom top[2];
    [mutating]
    void setTopVal(int x, int y, float value)
    {
        top[x].setVal(y, value);
    }
}

RWStructuredBuffer<Root> sb;

[shader("compute")]
[numthreads(1, 1, 1)]
void compute_main(uint3 tid: SV_DispatchThreadID)
{
    sb[0].setTopVal(1, 2, 100.0f);
}
```

We are now able to specialize the call to `setTopVal` into:
```
void compute_main(uint3 tid: SV_DispatchThreadID)
{
    setTopVal_specialized(0, 1, 2, 100.0f);
}

void setTopVal_specialized(int sbIdx, int x, int y, float value)
{
      Bottom_setVal_specialized(sbIdx, x, y, value);
}

void Bottom_setVal_specialized(int sbIdx, int x, int y, float value)
{
     sb[sbIdx].top[x].bigArray[y] = value;
}
```

And get rid of all unnecessary loads. Achieving this requires a
combination of function call specialization and buffer-load-defer pass.
The buffer-load-defer pass has been completely rewritten to be more
correct and avoid introducing redundant loads.

This PR also adds tests to make sure pointers, bindless handles, and
loads from structured buffer or constant buffers works as expected.

1 files changed, 58 insertions, 0 deletions

diff --git a/tests/optimization/buffer-load-defer-bindless.slang b/tests/optimization/buffer-load-defer-bindless.slang
new file mode 100644
index 000000000..2108d562c
--- /dev/null
+++ b/tests/optimization/buffer-load-defer-bindless.slang
@@ -0,0 +1,58 @@
+//TEST:SIMPLE(filecheck=CUDA): -target cuda -entry compute_main -stage compute
+//TEST:SIMPLE(filecheck=PTX): -target ptx -entry compute_main -stage compute
+
+//TEST:SIMPLE(filecheck=SPV): -target spirv
+
+// Check that we can specialize buffer loads through bindless handles, and
+// do not load big struct elements into registers unnecessarily.
+
+struct Bottom
+{
+    float bigArray[1024];
+    float bottomGetValue(int index) { return bigArray[index]; }
+}
+
+struct Middle
+{
+    Bottom bottom;
+    float middleGetValue(int index) { return bottom.bottomGetValue(index); }
+}
+
+struct Top
+{
+    StructuredBuffer<Middle>.Handle middle;
+
+    // Calling `middleGetValue` on `middle[0]` should not causing the entire `Middle`
+    // struct to be loaded into registers. Instead, we should be able to specialize
+    // `middleGetValue` to take a `StructuredBuffer<Middle>.Handle` and an `int`
+    // index, and recursively specialize `bottomGetValue` to only load the `Bottom.bigArray[index]` element.
+    float topGetValue(int index) { return middle[0].middleGetValue(index); }
+}
+
+struct Root
+{
+    Top top;
+}
+
+ConstantBuffer<Root> cb;
+
+RWStructuredBuffer<float> outputBuffer;
+
+// SPV: OpEntryPoint
+// SPV-NOT: OpLoad %Middle
+// SPV: %[[REG:[A-Za-z0-9_]+]] = OpLoad %float
+// SPV: OpStore {{.*}} %[[REG]]
+
+// Check that the generated CUDA code contains a specialized `bottomGetValue` function that has
+// the complete parameter list to access the `bigArray` element directly, without needing to load
+// the entire `Bottom` struct from the caller.
+//
+// CUDA-DAG: __device__ float Bottom_bottomGetValue{{.*}}(StructuredBuffer<Middle{{.*}}> {{.*}}, int {{.*}}, int {{.*}})
+// PTX: compute_main
+
+[shader("compute")]
+[numthreads(1, 1, 1)]
+void compute_main(uint3 tid: SV_DispatchThreadID)
+{
+    outputBuffer[0] = cb.top.topGetValue(0);
+}