DebugInfoManager::ConvertDebugGlobalToLocalVariable incorrectly converts a debug global variable to a debug local variable.

[AlexSabourinDev]

Summary

Hey all,

I believe this code in DebugInfoManager::ConvertDebugGlobalToLocalVariable incorrectly handles converting the operands from DebugGlobalVariable to DebugLocalVariable.

SPIRV-Tools/source/opt/debug_info_manager.cpp

Lines 775 to 778 in edc6895

	for (uint32_t i = dbg_global_var->NumInOperands() - 1;
	i >= kDebugLocalVariableOperandFlagsIndex; --i) {
	dbg_global_var->RemoveOperand(i);
	}

NumInOperands (typically returning 11) only represents the input operands count but kDebugLocalVariableOperandFlagsIndex is written in absolute operands (10).

Additionally, I think we want i > kDebugLocalVariableOperandFlagsIndex not i >= kDebugLocalVariableOperandFlagsIndex since we don't want to erase the flags operand.

I believe the correct code should look similar to:

for (uint32_t i = dbg_global_var->NumOperands() - 1; i > kDebugLocalVariableOperandFlagsIndex; --i) {
    dbg_global_var->RemoveOperand(i);
}

Repro Steps

I've included an example case using Shaderplayground and DXC:

https://shader-playground.timjones.io/b7f995367a738273718f6813ec847b8e

%37 = OpExtInst %void %1 DebugLocalVariable %14 %17 %23 %uint_6 %uint_20 %24 %uint_8 %uint_8

Notice that %37 represents the global Pi but notice the args operand is set to %uint_8 even though this is a global and not the 8th argument in a function.

When we disable optimizations we see the original variable declaration before the conversion.

https://shader-playground.timjones.io/c9f52e4e48b2de44f65e185b3e800a78

%48 = OpExtInst %void %1 DebugGlobalVariable %47 %19 %24 %uint_6 %uint_20 %25 %47 %Pi %uint_8

Conclusion

Hopefully this provides the information necessary. Let me know if there's anything else I can do to help.

Apologies for not directly submitting a patch. I don't have the appropriate testing environment set up.