libc: refactor psimd's __strong_alias logic a bit

This makes exporting partial TUs easier. Specifically, I'm trying to add
`wcslen` to ARM64.

It _could_ be better to swap to "headers where everything's implemented,
and `.cpp` files that force that have one-line implementations," but
this seems to work fine for now.

Bug: 452714218
Test: mma
Change-Id: I307158e26266ca2073285b54cb5b85c35ae99b76

libc/portable-simd/memchr.cpp

diff --git a/libc/portable-simd/memchr.cpp b/libc/portable-simd/memchr.cpp
index 154a2c4..96589a3 100644
--- a/libc/portable-simd/memchr.cpp
+++ b/libc/portable-simd/memchr.cpp
@@ -419,3 +419,9 @@
       static_cast<const wchar_t*>(portable_simd::memchr_vectorized<WmemchrTraits>(
           reinterpret_cast<const WmemchrTraits::CharType*>(ptr), ch, count)));
 }
+
+#if defined(__x86_64__)
+PSIMD_MAYBE_STRONG_ALIAS(memchr);
+PSIMD_MAYBE_STRONG_ALIAS(memrchr);
+PSIMD_MAYBE_STRONG_ALIAS(wmemchr);
+#endif

libc/portable-simd/portable_simd_detail.h

diff --git a/libc/portable-simd/portable_simd_detail.h b/libc/portable-simd/portable_simd_detail.h
index 294f977..41f2eb6 100644
--- a/libc/portable-simd/portable_simd_detail.h
+++ b/libc/portable-simd/portable_simd_detail.h
@@ -89,14 +89,23 @@
 // should be put on `PSIMD_LIBC_FUNCTION`.
 #define PSIMD_FLATTEN __attribute__((__flatten__))
 
+// Given e.g., `strlen`, gives back the portable_simd name for the current
+// compilation, e.g., `portable_simd_strlen_avx2`.
+#define PSIMD_CONCAT1(x, y) x##y
+#define PSIMD_CONCAT(x, y) PSIMD_CONCAT1(x, y)
+#define PSIMD_LIBC_FUNCTION_NAME(libc_name) \
+  PSIMD_CONCAT(PSIMD_CONCAT(portable_simd_, libc_name), PSIMD_EXPORT_SUFFIX)
+
 // If PSIMD_ADD_LIBC_ALIASES is defined, we'll emit strong aliases for each
 // function to its corresponding libc function. For example, if this TU defines
 // a portable-simd version of `strlen` for SSE, `-DPSIMD_ADD_LIBC_ALIASES` will
 // emit a definition of `strlen` as well as `portable_simd_strlen_sse`.
 #if defined(PSIMD_ADD_LIBC_ALIASES)
-#define PSIMD_MAYBE_STRONG_ALIAS(libc_name, impl_func) __strong_alias(libc_name, impl_func)
+#define PSIMD_MAYBE_STRONG_ALIAS1(libc_name, impl_name) __strong_alias(libc_name, impl_name)
+#define PSIMD_MAYBE_STRONG_ALIAS(libc_name) \
+  PSIMD_MAYBE_STRONG_ALIAS1(libc_name, PSIMD_LIBC_FUNCTION_NAME(libc_name))
 #else
-#define PSIMD_MAYBE_STRONG_ALIAS(libc_name, impl_func)
+#define PSIMD_MAYBE_STRONG_ALIAS(libc_name)
 #endif
 
 // Attributes to place on functions that we 'export', AKA are designed to be
@@ -112,15 +121,10 @@
 // `avx2` depends on the SIMD instructions we're targeting). If
 // `PSIMD_ADD_LIBC_ALIASES` is enabled, it also adds a strong alias for
 // `strlen = portable_simd_strlen_avx2`.
-#define PSIMD_CONCAT1(x, y) x##y
-#define PSIMD_CONCAT(x, y) PSIMD_CONCAT1(x, y)
 #define PSIMD_LIBC_FUNCTION_IMPL(ret_ty, libc_name, full_name, ...) \
-  PSIMD_MAYBE_STRONG_ALIAS(libc_name, full_name)                    \
   PSIMD_FLATTEN __attribute__((__hot__)) ret_ty full_name(__VA_ARGS__)
 #define PSIMD_LIBC_FUNCTION(ret_ty, libc_name, ...) \
-  PSIMD_LIBC_FUNCTION_IMPL(                         \
-      ret_ty, libc_name,                            \
-      PSIMD_CONCAT(PSIMD_CONCAT(portable_simd_, libc_name), PSIMD_EXPORT_SUFFIX), __VA_ARGS__)
+  PSIMD_LIBC_FUNCTION_IMPL(ret_ty, libc_name, PSIMD_LIBC_FUNCTION_NAME(libc_name), __VA_ARGS__)
 
 namespace portable_simd {
 

libc/portable-simd/strlen.cpp

diff --git a/libc/portable-simd/strlen.cpp b/libc/portable-simd/strlen.cpp
index 14f9a99..486bad5 100644
--- a/libc/portable-simd/strlen.cpp
+++ b/libc/portable-simd/strlen.cpp
@@ -239,3 +239,8 @@
   return portable_simd::strlen_vectorized<WcslenTraits>(
       reinterpret_cast<const WcslenTraits::CharType*>(s));
 }
+
+#if defined(__x86_64__)
+PSIMD_MAYBE_STRONG_ALIAS(strlen);
+PSIMD_MAYBE_STRONG_ALIAS(wcslen);
+#endif