[llvm] [SPIR-V] Avoid rescanning use lists to check for Block decoration (PR #211320)

[Arseniy Obolenskiy via llvm-commits]

aobolensk wrote:

I have made some experiment on that:

```
#!/usr/bin/env python3
# Generates an .ll module that stresses SPIRVGlobalRegistry::isResourceType
# on a struct type (%BigStruct) referenced from many distinct array types.
# Each array type requires a fresh isResourceType(%BigStruct) check when
# deciding whether to add an ArrayStride decoration, so with N array types
# the check runs N times against the same struct's type register.
import sys

num_fields = int(sys.argv[1]) if len(sys.argv) > 1 else 10
num_arrays = int(sys.argv[2]) if len(sys.argv) > 2 else 20000

field_tys = ", ".join(["i32"] * num_fields)
lines = [
    'target datalayout = "e-i64:64-v16:16-v24:32-v32:32-v48:64-v96:128-v192:256-v256:256-v512:512-v1024:1024-n8:16:32:64-G1"',
    "",
    f"%BigStruct = type <{{ {field_tys} }}>",
    "",
    "%Holder = type <{ " + ", ".join(f"[{i + 1} x %BigStruct]" for i in range(num_arrays)) + " }>",
    "",
    "@pc = external hidden addrspace(13) externally_initialized global %Holder, align 1",
    "",
    "define void @main() #1 {",
    "entry:",
    "  ret void",
    "}",
    "",
    'attributes #1 = { convergent noinline norecurse optnone "hlsl.numthreads"="1,1,1" "hlsl.shader"="compute" }',
    "",
]
print("\n".join(lines))
```

Usage: `python3 gen_bench.py <fields-per-struct> <num-array-types> > bench.ll`, then `llc -O0 -mtriple=spirv-vulkan-unknown bench.ll -o /dev/null`

Results:
| N (array types over shared struct) | Before | After | Speedup |
|-----------------------------------:|-------:|------:|--------:|
|                              5,000 |  0.07s | 0.03s |    2.3× |
|                             10,000 |  0.24s | 0.05s |    4.8× |
|                             20,000 |  0.81s | 0.11s |    7.4× |
|                             40,000 |  3.07s | 0.25s |   12.3× |


For sure, this situation is quite artificial (I hardly imagine such cases)

https://github.com/llvm/llvm-project/pull/211320