Implement goto statements and labels

[jserv]

Description

shecc lacks support for goto statements and labels, which are part of the C standard. While goto is often discouraged, it's essential for certain patterns like error handling in systems code and is required for full C compliance.

Current Behavior

void func() {
    goto cleanup;  // Error: "goto statement not supported"
cleanup:           // Error: labels not recognized
    return;
}

Expected Behavior

Full support for labels and goto statements:

• Forward and backward jumps
• Labels in any statement context
• Proper scope handling
• Integration with control flow graph

Implementation Requirements

1. Parser Changes

• Recognize label syntax (identifier:)
• Parse goto identifier; statements
• Track label definitions and references
• Handle forward references to undefined labels

2. Symbol Table

• Add label namespace separate from variables
• Store label locations in IR
• Validate all gotos reference defined labels

3. Code Generation

• Map labels to basic block boundaries
• Generate unconditional branch instructions
• Ensure proper control flow in CFG

4. Semantic Checks

• Detect duplicate label definitions
• Error on undefined labels after parsing
• Scope validation (labels are function-scoped)

Test Cases

// Basic goto
int basic() {
    int x = 0;
    goto skip;
    x = 1;
skip:
    return x;  // Should return 0
}

// Error handling pattern
int error_handling() {
    void *p1 = malloc(100);
    if (!p1) goto cleanup;
    
    void *p2 = malloc(200);
    if (!p2) goto cleanup_p1;
    
    // Use resources
    free(p2);
cleanup_p1:
    free(p1);
cleanup:
    return -1;
}

// Forward reference
int forward() {
    goto end;  // Forward reference
    return 1;
end:
    return 0;
}

// Loop with goto
int loop() {
    int i = 0;
start:
    if (i++ < 10)
        goto start;
    return i;
}

Complexity

MEDIUM - Requires parser changes and CFG integration

Benefits

• Full C standard compliance
• Enables certain system programming patterns
• Required for porting legacy code
• Useful for generated code patterns

Notes

• Consider computed goto (GNU extension) as future enhancement
• Ensure goto doesn't cross variable initialization
• Document interaction with other control flow