compiler: use memcmp to compare strings #5149
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This replaces our runtime stringEqual and stringLess functions with calls to libc's memcmp. This has a few advantages: 1. Memory comparison functions are not duplicated between Go and C 2. LLVM can optimize the compare-to-empty case by itself, so OptimizeStringEqual is no longer needed 3. LLVM can rewrite small constant-length memcmp operations into direct loads + compares 4. Compares to constants are generally compiled into simpler IR The downside of this is that all of the comparison logic must be handled by the compiler frontend. For string equality checks, we only need to compare the lengths and memcmp if they are equal. String order checks are messier, since we need to: 1. Find the minimum length 2. Call memcmp with the minimum length 3. Compare the lengths 4. Merge the length comparison with the memory comparison
dgryski
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Jan 5, 2026
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With the exception of the signed/unsigned length question this is probably good to merge. And if it's broken we'll find out soon enough :D
| // Calculate the minimum of the two string lengths. | ||
| lhsLen := b.CreateExtractValue(lhs, 1, "strlt.lhs.len") | ||
| rhsLen := b.CreateExtractValue(rhs, 1, "strlt.rhs.len") | ||
| minFnName := "llvm.umin.i" + strconv.Itoa(b.uintptrType.IntTypeWidth()) |
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The top bit is never set so they are both, although AVR is slightly awkward since the length is actually uintptr.
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This replaces our runtime stringEqual and stringLess functions with calls to libc's memcmp. This has a few advantages:
The downside of this is that all of the comparison logic must be handled by the compiler frontend. For string equality checks, we only need to compare the lengths and memcmp if they are equal. String order checks are messier, since we need to: