diff --git a/libc/CMakeLists.txt b/libc/CMakeLists.txt
--- a/libc/CMakeLists.txt
+++ b/libc/CMakeLists.txt
@@ -22,6 +22,9 @@
 # Defines LIBC_TARGET_ARCHITECTURE and associated macros.
 include(LLVMLibCArchitectures)
 
+# Flags to pass down to the compiler while building the libc functions.
+set(LIBC_COMPILE_OPTIONS_DEFAULT "" CACHE STRING "Architecture to tell clang to optimize for (e.g. -march=... or -mcpu=...)")
+
 # Check --print-resource-dir to find the compiler resource dir if this flag
 # is supported by the compiler.
 execute_process(
diff --git a/libc/cmake/modules/LLVMLibCLibraryRules.cmake b/libc/cmake/modules/LLVMLibCLibraryRules.cmake
--- a/libc/cmake/modules/LLVMLibCLibraryRules.cmake
+++ b/libc/cmake/modules/LLVMLibCLibraryRules.cmake
@@ -81,10 +81,8 @@
     STATIC
     ${objects}
   )
-  set_target_properties(
-    ${target_name}
-    PROPERTIES
-      ARCHIVE_OUTPUT_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}
+  set_target_properties(${target_name}  PROPERTIES ARCHIVE_OUTPUT_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR})
+  target_compile_options(${target_name} PRIVATE ${LIBC_COMPILE_OPTIONS_DEFAULT}
   )
 endfunction(add_entrypoint_library)
 
@@ -112,18 +110,10 @@
     SHARED
     ${obj_files}
   )
-  set_target_properties(${target_name} PROPERTIES LIBRARY_OUTPUT_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR})
-
-  target_link_libraries(
-    ${target_name}
-    -nostdlib -lc -lm
-  )
-
-  set_target_properties(
-    ${target_name}
-    PROPERTIES
-      LINKER_LANGUAGE "C"
-  )
+  set_target_properties(${target_name}  PROPERTIES LIBRARY_OUTPUT_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR})
+  target_link_libraries(${target_name}  -nostdlib -lc -lm)
+  set_target_properties(${target_name}  PROPERTIES LINKER_LANGUAGE "C")
+  target_compile_options(${target_name} PRIVATE ${LIBC_COMPILE_OPTIONS_DEFAULT})
 endfunction(add_redirector_library)
 
 set(HDR_LIBRARY_TARGET_TYPE "HDR_LIBRARY")
diff --git a/libc/cmake/modules/LLVMLibCObjectRules.cmake b/libc/cmake/modules/LLVMLibCObjectRules.cmake
--- a/libc/cmake/modules/LLVMLibCObjectRules.cmake
+++ b/libc/cmake/modules/LLVMLibCObjectRules.cmake
@@ -37,6 +37,10 @@
       ${LIBC_SOURCE_DIR}
       ${LIBC_BUILD_DIR}
   )
+  target_compile_options(
+    ${fq_target_name}
+    PRIVATE ${LIBC_COMPILE_OPTIONS_DEFAULT}
+  )
   if(ADD_OBJECT_COMPILE_OPTIONS)
     target_compile_options(
       ${fq_target_name}
@@ -148,7 +152,7 @@
     message(FATAL_ERROR "`add_entrypoint_object` rule requires HDRS to be specified.")
   endif()
 
-  set(common_compile_options -fpie ${LLVM_CXX_STD_default} -ffreestanding ${ADD_ENTRYPOINT_OBJ_COMPILE_OPTIONS})
+  set(common_compile_options -fpie ${LLVM_CXX_STD_default} -ffreestanding ${LIBC_COMPILE_OPTIONS_DEFAULT} ${ADD_ENTRYPOINT_OBJ_COMPILE_OPTIONS})
   set(internal_target_name ${fq_target_name}.__internal__)
   set(include_dirs ${LIBC_BUILD_DIR}/include ${LIBC_SOURCE_DIR} ${LIBC_BUILD_DIR})
   get_fq_deps_list(fq_deps_list ${ADD_ENTRYPOINT_OBJ_DEPENDS})
@@ -277,6 +281,6 @@
   )
   target_compile_options(
     ${target_name}
-    BEFORE PRIVATE -fPIC
+    BEFORE PRIVATE -fPIC ${LIBC_COMPILE_OPTIONS_DEFAULT}
   )
 endfunction(add_redirector_object)
diff --git a/libc/cmake/modules/LLVMLibCTestRules.cmake b/libc/cmake/modules/LLVMLibCTestRules.cmake
--- a/libc/cmake/modules/LLVMLibCTestRules.cmake
+++ b/libc/cmake/modules/LLVMLibCTestRules.cmake
@@ -127,6 +127,10 @@
       ${LIBC_BUILD_DIR}
       ${LIBC_BUILD_DIR}/include
   )
+  target_compile_options(
+    ${fq_target_name}
+    PRIVATE ${LIBC_COMPILE_OPTIONS_DEFAULT}
+  )
   if(LIBC_UNITTEST_COMPILE_OPTIONS)
     target_compile_options(
       ${fq_target_name}