[libc] add PREFER_GENERIC flag (#73744)

There are some basic vectorization features in standard architecture
specifications. Such as SSE/SSE2 for x86-64, or NEON for aarch64. Even
though such features are almost always available, we still need some
methods to test fallback routines without any vectorization.

Previous attempt in hsearch adds a DISABLE_SSE2_OPT flag that tries to
compile the code with -mno-sse2 in order to test specific table scanning
routines. However, it turns out that such flag may have some unwanted
side effects hindering portability.

This PR introduces PREFER_GENERIC as an alternative. When a target is
built with PREFER_GENERIC, cmake will define a macro
__LIBC_PREFER_GENERIC such that developers can selectively choose the
fallback routine based on the macro.

Author: SchrodingerZhu

libc/cmake/modules/LLVMLibCFlagRules.cmake

@@ -132,8 +132,9 @@ endfunction(get_fq_dep_list_without_flag)
 # Special flags
 set(FMA_OPT_FLAG "FMA_OPT")
 set(ROUND_OPT_FLAG "ROUND_OPT")
-# SSE2 is the baseline for x86_64, so we add a negative flag to disable it if needed.
-set(DISABLE_SSE2_OPT_FLAG "DISABLE_SSE2_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")
 
 # 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
@@ -145,8 +146,3 @@ endif()
 if(NOT(LIBC_TARGET_ARCHITECTURE_IS_X86 AND (LIBC_CPU_FEATURES MATCHES "SSE4_2")))
   set(SKIP_FLAG_EXPANSION_ROUND_OPT TRUE)
 endif()
-
-# Skip DISABLE_SSE2_OPT flag for targets that don't support SSE2.
-if(NOT(LIBC_TARGET_ARCHITECTURE_IS_X86 AND (LIBC_CPU_FEATURES MATCHES "SSE2")))
-  set(SKIP_FLAG_EXPANSION_DISABLE_SSE2_OPT TRUE)
-endif()

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 ${DISABLE_SSE2_OPT_FLAG} no_sse2)
-  if(${no_sse2} LESS 0)
-    list(FIND flags "${DISABLE_SSE2_OPT_FLAG}__ONLY" no_sse2)
+  list(FIND flags ${PREFER_GENERIC_FLAG} prefer_generic)
+  if(${prefer_generic} LESS 0)
+    list(FIND flags "${PREFER_GENERIC_FLAG}__ONLY" prefer_generic)
   endif()
-  if((${no_sse2} GREATER -1) AND (LIBC_CPU_FEATURES MATCHES "SSE2"))
-    set(DISABLE_SSE2_FLAG TRUE)
+  if(${prefer_generic} GREATER -1)
+    set(ADD_PREFER_GENERIC_FLAG TRUE)
   endif()
 
   set(compile_options ${LIBC_COMPILE_OPTIONS_DEFAULT} ${ARGN})
@@ -66,17 +66,17 @@ function(_get_common_compile_options output_var flags)
     if(ADD_SSE4_2_FLAG)
       list(APPEND compile_options "-msse4.2")
     endif()
-    if(DISABLE_SSE2_FLAG)
-      list(APPEND compile_options "-mno-sse2")
+    if(ADD_PREFER_GENERIC_FLAG)
+      list(APPEND compile_options "-D__LIBC_PREFER_GENERIC")
     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(DISABLE_SSE2_FLAG)
-      list(APPEND compile_options "/arch:SSE")
+    if(ADD_PREFER_GENERIC_FLAG)
+      list(APPEND compile_options "/D__LIBC_PREFER_GENERIC")
     endif()
   endif()
   if (LIBC_TARGET_ARCHITECTURE_IS_GPU)

libc/src/__support/HashTable/CMakeLists.txt

@@ -1,11 +1,9 @@
-# TODO: `DISABLE_SSE2_OPT` does not quite work yet.
-# We will investigate a better way of feature flag control.
 add_header_library(
   bitmask
   HDRS
     bitmask.h
   FLAGS
-    DISABLE_SSE2_OPT
+    PREFER_GENERIC
   DEPENDS
     libc.src.__support.bit
     libc.src.__support.macros.properties.cpu_features

libc/src/__support/HashTable/bitmask.h

@@ -85,7 +85,7 @@ template <class BitMask> struct IteratableBitMaskAdaptor : public BitMask {
 } // namespace internal
 } // namespace LIBC_NAMESPACE
 
-#if defined(LIBC_TARGET_CPU_HAS_SSE2)
+#if defined(LIBC_TARGET_CPU_HAS_SSE2) && !defined(__LIBC_PREFER_GENERIC)
 #include "sse2/bitmask_impl.inc"
 #else
 #include "generic/bitmask_impl.inc"