opt: invalid SPIR-V produced: block <ID> 21[%21] exits the selection headed by <ID> 11[%11], but not via a structured exit

[paulthomson]

bug_report.zip

spiv-opt produces invalid SPIR-V. Output:

error: line 34: block <ID> 21[%21] exits the selection headed by <ID> 11[%11], but not via a structured exit
  %21 = OpLabel

error: line 0: Validation failed after pass merge-blocks

I manually checked the SPIR-V. I believe there is a branch to a continue target that also illegally exits a selection.

Tool versions:

• glslangValidator commit hash: 40c16ec0b3ad03fc170f1369a58e7bbe662d82cd
• spirv-opt commit hash: 4a00a80c40484a6f6f72f48c9d34943cf8f180d4

To reproduce:

glslangValidator -V shader.frag -o shader.frag.spv

spirv-opt shader.frag.spv -o temp.spv --validate-after-all --reduce-load-size --scalar-replacement=100 --simplify-instructions --redundancy-elimination --merge-blocks

The following shader files are included in the attached archive:

• 0_glsl/shader.frag
• 1_spirv/shader.frag.spv
• 1_spirv_asm/shader.frag.asm
• 2_spirv_opt/shader.frag.spv (this is after optimization)

Issue found using GraphicsFuzz.

[paulthomson]

This is not a bug. We apparently need to run --eliminate-dead-branches before --merge-blocks. We will update the fuzzer.