[libc] change PREFER_GENERIC to EXPLICIT_SIMD_OPT (#79486)

fixes #79474.

Author: SchrodingerZhu

libc/cmake/modules/LLVMLibCFlagRules.cmake

@@ -132,16 +132,21 @@ endfunction(get_fq_dep_list_without_flag)
 # Special flags
 set(FMA_OPT_FLAG "FMA_OPT")
 set(ROUND_OPT_FLAG "ROUND_OPT")
-# This flag will define macros that gated away vectorization code such that
-# one can always test the fallback generic code path.
-set(PREFER_GENERIC_FLAG "PREFER_GENERIC")
+# This flag controls whether we use explicit SIMD instructions or not.
+set(EXPLICIT_SIMD_OPT_FLAG "EXPLICIT_SIMD_OPT")
 
 # Skip FMA_OPT flag for targets that don't support fma.
 if(NOT((LIBC_TARGET_ARCHITECTURE_IS_X86 AND (LIBC_CPU_FEATURES MATCHES "FMA")) OR
        LIBC_TARGET_ARCHITECTURE_IS_RISCV64))
   set(SKIP_FLAG_EXPANSION_FMA_OPT TRUE)
 endif()
 
+# Skip EXPLICIT_SIMD_OPT flag for targets that don't support SSE2.
+# Note: one may want to revisit it if they want to control other explicit SIMD
+if(NOT(LIBC_TARGET_ARCHITECTURE_IS_X86 AND (LIBC_CPU_FEATURES MATCHES "SSE2")))
+  set(SKIP_FLAG_EXPANSION_EXPLICIT_SIMD_OPT TRUE)
+endif()
+
 # Skip ROUND_OPT flag for targets that don't support SSE 4.2.
 if(NOT(LIBC_TARGET_ARCHITECTURE_IS_X86 AND (LIBC_CPU_FEATURES MATCHES "SSE4_2")))
   set(SKIP_FLAG_EXPANSION_ROUND_OPT TRUE)

libc/cmake/modules/LLVMLibCObjectRules.cmake

@@ -18,12 +18,12 @@ function(_get_common_compile_options output_var flags)
     set(ADD_SSE4_2_FLAG TRUE)
   endif()
 
-  list(FIND flags ${PREFER_GENERIC_FLAG} prefer_generic)
-  if(${prefer_generic} LESS 0)
-    list(FIND flags "${PREFER_GENERIC_FLAG}__ONLY" prefer_generic)
+  list(FIND flags ${EXPLICIT_SIMD_OPT_FLAG} explicit_simd)
+  if(${explicit_simd} LESS 0)
+    list(FIND flags "${EXPLICIT_SIMD_OPT_FLAG}__ONLY" explicit_simd)
   endif()
-  if(${prefer_generic} GREATER -1)
-    set(ADD_PREFER_GENERIC_FLAG TRUE)
+  if(${explicit_simd} GREATER -1)
+    set(ADD_EXPLICIT_SIMD_OPT_FLAG TRUE)
   endif()
 
   set(compile_options ${LIBC_COMPILE_OPTIONS_DEFAULT})
@@ -73,17 +73,17 @@ function(_get_common_compile_options output_var flags)
     if(ADD_SSE4_2_FLAG)
       list(APPEND compile_options "-msse4.2")
     endif()
-    if(ADD_PREFER_GENERIC_FLAG)
-      list(APPEND compile_options "-D__LIBC_PREFER_GENERIC")
+    if(ADD_EXPLICIT_SIMD_OPT_FLAG)
+      list(APPEND compile_options "-D__LIBC_EXPLICIT_SIMD_OPT")
     endif()
   elseif(MSVC)
     list(APPEND compile_options "/EHs-c-")
     list(APPEND compile_options "/GR-")
     if(ADD_FMA_FLAG)
       list(APPEND compile_options "/arch:AVX2")
     endif()
-    if(ADD_PREFER_GENERIC_FLAG)
-      list(APPEND compile_options "/D__LIBC_PREFER_GENERIC")
+    if(ADD_EXPLICIT_SIMD_OPT_FLAG)
+      list(APPEND compile_options "/D__LIBC_EXPLICIT_SIMD_OPT")
     endif()
   endif()
   if (LIBC_TARGET_ARCHITECTURE_IS_GPU)

libc/src/__support/HashTable/CMakeLists.txt

@@ -3,7 +3,7 @@ add_header_library(
   HDRS
     bitmask.h
   FLAGS
-    PREFER_GENERIC
+    EXPLICIT_SIMD_OPT
   DEPENDS
     libc.src.__support.common
     libc.src.__support.CPP.bit

libc/src/__support/HashTable/bitmask.h

@@ -83,7 +83,7 @@ template <class BitMask> struct IteratableBitMaskAdaptor : public BitMask {
 } // namespace internal
 } // namespace LIBC_NAMESPACE
 
-#if defined(LIBC_TARGET_CPU_HAS_SSE2) && !defined(__LIBC_PREFER_GENERIC)
+#if defined(LIBC_TARGET_CPU_HAS_SSE2) && defined(__LIBC_EXPLICIT_SIMD_OPT)
 #include "sse2/bitmask_impl.inc"
 #else
 #include "generic/bitmask_impl.inc"