ssa: fix zero-value map indexing

[zhouguangyuan0718]

Summary

• represent zero-value maps as null *runtime.Map pointers in SSA zero-value lowering
• avoid constant-extracting array elements from nil-backed values so zero-value map indexing stays on the normal load path
• add cl/_testgo/mapzero to cover the runtime output and the FileCheck expectation

[gemini-code-assist]

Code Review

This pull request introduces a test case for zero-length array indexing and adjusts the handling of map zero values in the SSA builder. Specifically, it ensures that map zero values are represented as pointers and removes a constant extraction optimization in ssa/datastruct.go to ensure consistent behavior. A review comment suggests improving the robustness of the LLVM IR check by using wildcards instead of hardcoded register names.

[fennoai]

Review Summary

The fix is correct: zero-value maps should be represented as null *runtime.Map pointers (matching how ssa/type.go types maps as llvm.PointerType(p.rtMap(), 0)), not as zero-initialized Map structs. The ConstExtractElement removal is also warranted since it bypassed checkIndex, allowing UB on zero-length arrays. The performance impact is negligible — LLVM's mem2reg/SROA passes will optimize the alloc+store fallback, and constfold handles the constant case automatically.

Two minor notes on the test below.

[fennoai]

Nit (fragile CHECK pattern): The hardcoded %21 register will break if any unrelated compiler change shifts the IR instruction count. The tpinst test uses %{{[0-9]+}} for exactly this reason. Consider:

// CHECK: call ptr @"github.com/goplus/llgo/runtime/internal/runtime.MapAccess1"(ptr @"map[_llgo_int]_llgo_int", ptr null, ptr %{{[0-9]+}})

The key property being verified is ptr null as the map argument — the register number of the key pointer is not relevant to the fix.

[fennoai]

Nit (readability): This expression is dense and tests two things at once (array bounds check + nil map representation). A short comment would help future readers:

// Index a zero-length array literal to get a nil map, then index that nil map.
// The array index panics with "index out of range" before the map access.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.50%. Comparing base (9b0889e) to head (313211f).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1835      +/-   ##
==========================================
+ Coverage   88.48%   88.50%   +0.02%     
==========================================
  Files          50       50              
  Lines       14428    14426       -2     
==========================================
+ Hits        12766    12768       +2     
+ Misses       1441     1438       -3     
+ Partials      221      220       -1     

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