[SPIR-V] DXC crashes when declaring a variable in multiple case block

[shobomaru]

Description
LLVM assertion failed.

Steps to Reproduce
dxc -T cs_6_0 -spirv path/to/file.hlsl

HLSL code:

uint Get(uint a)
{
        [branch]
        switch (a)
        {
        case 0: {
                uint d = a + 1;
                return a + 1;
        }
        case 1: {
                uint d1 = a + 2;
                return a + 2;
        }
        }
        return 0;
}

RWStructuredBuffer<uint> Out;
[numthreads(1, 1, 1)]
void main()
{
        uint a = Out[0];
        uint v = Get(a);
        Out[0] = v;
}

If change the code in one of the following ways, the issue is not occurred.

• Remove [branch]
• Remove uint d = a + 1; or uint d1 = a + 2; (in other words, only declaring one variable in the switch statement)

Actual Behavior

Assertion failed: (astDecls[var].instr == nullptr), function createFnVar, file DeclResultIdMapper.cpp, line 1106.

Environment

• DXC version: libdxcompiler.dylib: 1.8(dev;4640-45018c75)
• Host Operating System: Windows 11 23H2 and macOS 14.5

[Keenuts]

Thanks for the issue!

Repro:

// RUN: %dxc -T cs_6_6 %s -E main -fcgl -spirv | FileCheck %s

[numthreads(1, 1, 1)]
void main()
{
  uint a = 0;

  [branch]
  switch (a) {
    default: return;
    case 1: {
      uint d1;
    }
  }
}

Looks like this is caused by our code to map variable to declaration: we process 2 VarDecl nodes that declare the same variable. And there is an assert for that.
The base AST is fine:

  |   `-SwitchStmt 0x55fe2c490258 <line:9:3, line:14:3>
  |     |-<<<NULL>>>
  |     |-ImplicitCastExpr 0x55fe2c490240 <line:9:11> 'uint':'unsigned int' <LValueToRValue>
  |     | `-DeclRefExpr 0x55fe2c490200 <col:11> 'uint':'unsigned int' lvalue Var 0x55fe2c490140 'a' 'uint':'unsigned int'
  |     `-CompoundStmt 0x55fe2c4903e0 <col:14, line:14:3>
  |       |-DefaultStmt 0x55fe2c490298 <line:10:5, col:14>
  |       | `-ReturnStmt 0x55fe2c490280 <col:14>
  |       `-CaseStmt 0x55fe2c4902d8 <line:11:5, line:13:5>
  |         |-ImplicitCastExpr 0x55fe2c490408 <line:11:10> 'uint':'unsigned int' <IntegralCast>
  |         | `-IntegerLiteral 0x55fe2c4902b8 <col:10> 'literal int' 1
  |         |-<<<NULL>>>
  |         `-CompoundStmt 0x55fe2c4903a8 <col:13, line:13:5>
  |           `-DeclStmt 0x55fe2c490390 <line:12:7, col:14>
  |             `-VarDecl 0x55fe2c490320 <col:7, col:12> col:12 d1 'uint':'unsigned int'

But because of the [Branch] tag, we have the following AST we flatten the Switch, and seems like we do something wrong there.

IfStmt 0x555555a65920
|-<<<NULL>>>
|-BinaryOperator 0x555555a65958 'bool' '=='
| |-ImplicitCastExpr 0x555555a656a0 'uint':'unsigned int' <LValueToRValue>
| | `-DeclRefExpr 0x555555a65660 'uint':'unsigned int' lvalue Var 0x555555a655a0 'a' 'uint':'unsigned int'
| `-ImplicitCastExpr 0x555555a65868 'uint':'unsigned int' <IntegralCast>
|   `-IntegerLiteral 0x555555a65718 'literal int' 1
|-CompoundStmt 0x555555a65900
| `-DeclStmt 0x555555a657f0
|   `-VarDecl 0x555555a65780  d1 'uint':'unsigned int'
`-CompoundStmt 0x555555a658d8
  |-ReturnStmt 0x555555a656e0
  `-DeclStmt 0x555555a657f0
    `-VarDecl 0x555555a65780  d1 'uint':'unsigned int'