[llvm-bugs] [Bug 92135] Clang Generating Invalid SPIRV

[LLVM Bugs via llvm-bugs]

Issue	92135
Summary	Clang Generating Invalid SPIRV
Labels	clang
Assignees
Reporter	Calandracas606

    I am trying to compile C++ for OpenCL kernels to spirv, however clang is creating invalid spirv (verified with `spirv-val`)

clang 18.1.4 built from source on x86_64 linux

I am using the `-O` flag, which appears to be experimental:
https://github.com/KhronosGroup/OpenCL-Guide/blob/main/chapters/os_tooling.md

simple reproducer `kernel.clcpp`:
```cpp
void sample(read_only image2d_t in_img, write_only image2d_t out_img) {

  const int col = get_global_id(0);
  const int row = get_global_id(1);

  const sampler_t samplerA = CLK_NORMALIZED_COORDS_FALSE |
 CLK_ADDRESS_CLAMP_TO_EDGE | CLK_FILTER_NEAREST;

  float4 px = {0.0F, 0.0F, 0.0F, 1.0F};
  for (int i = 0; i < 3; ++i) {
    for (int j = 0; j < 3; ++j) {
      float4 val = read_imagef(in_img, samplerA, int2{col + i, row + i});
      px += val.xxxx;
    }
  }

 write_imagef(out_img, int2{col, row}, px);
};
```
I try to compile with:

```
clang -c -target spirv64 -O -o sample.spv kernel.clcpp
spirv-val sample.spv
```

i get the output:
```
error: line 51: Block '17[%17]' is already a merge block for another header
  %_Z6sample14ocl_image2d_ro14ocl_image2d_wo = OpFunction %void None %9
```
when I remove the `-O` flag, `spirv-val` no longer reports an error

when i manually invoke `llvm-spirv` and `spirv-opt` there is no error:
```
clang -c -target spirv64 -emit-llvm -o sample2.bc kernel.clcpp
llvm-spirv sample2.bc -o sample2.spv
spirv-opt sample2.spv -o sample2_opt.spv
spirv-val sample2_opt.spv
```

please let me know if this is not the correct area to report this bug, so I can report it elsewhere

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