Adding an option to disable TLS (#632)

Signed-off-by: Matthew A Johnson <matjoh@microsoft.com>

CMakeLists.txt

@@ -26,6 +26,7 @@ option(SNMALLOC_NO_REALLOCARR "Build without reallocarr exported" ON)
 option(SNMALLOC_LINK_ICF "Link with Identical Code Folding" ON)
 option(SNMALLOC_IPO "Link with IPO/LTO support" OFF)
 option(SNMALLOC_BENCHMARK_INDIVIDUAL_MITIGATIONS "Build tests and ld_preload for individual mitigations" OFF)
+option(SNMALLOC_ENABLE_DYNAMIC_LOADING "Build such that snmalloc can be dynamically loaded. This is not required for LD_PRELOAD, and will harm performance if enabled." OFF)
 # Options that apply only if we're not building the header-only library
 cmake_dependent_option(SNMALLOC_RUST_SUPPORT "Build static library for rust" OFF "NOT SNMALLOC_HEADER_ONLY_LIBRARY" OFF)
 cmake_dependent_option(SNMALLOC_STATIC_LIBRARY "Build static libraries" ON "NOT SNMALLOC_HEADER_ONLY_LIBRARY" OFF)
@@ -396,7 +397,8 @@ if(NOT SNMALLOC_HEADER_ONLY_LIBRARY)
         target_compile_options(${name} PRIVATE -mprfchw)
       endif()
       # Static TLS model is unsupported on Haiku.
-      if (NOT CMAKE_SYSTEM_NAME STREQUAL "Haiku")
+      if ((NOT CMAKE_SYSTEM_NAME STREQUAL "Haiku") AND (NOT SNMALLOC_ENABLE_DYNAMIC_LOADING))
+        message(STATUS "snmalloc: Using static TLS model")
         target_compile_options(${name} PRIVATE -ftls-model=initial-exec)
         target_compile_options(${name} PRIVATE $<$<BOOL:${SNMALLOC_CI_BUILD}>:-g>)
       endif()