Add -fno-builtin to dlmalloc / math library build (emscripten-core#6289)

aheejin · web-flow · commit 89454b10e5c6 · 2018-03-01T18:48:45.000-08:00
Without -fno-builtin, LLVM can optimize away or convert calls to library
functions to something else based on assumptions that they behave
exactly like the standard library. This can cause unexpected bugs when
we use our custom standard library.
diff --git a/tools/system_libs.py b/tools/system_libs.py
@@ -132,7 +132,11 @@ def create_libc(libname):
               break
           if not cancel:
             libc_files.append(os.path.join(musl_srcdir, dirpath, f))
-    args = ['-Os']
+    # Without -fno-builtin, LLVM can optimize away or convert calls to library
+    # functions to something else based on assumptions that they behave exactly
+    # like the standard library. This can cause unexpected bugs when we use our
+    # custom standard library. The same for other libc/libm builds.
+    args = ['-Os', '-fno-builtin']
     if shared.Settings.USE_PTHREADS:
       args += ['-s', 'USE_PTHREADS=1']
       assert '-mt' in libname
@@ -196,7 +200,7 @@ def create_wasm_libc(libname):
                  'atan2.c', 'atan2f.c', 'atan2l.c', 'exp.c', 'expf.c', 'expl.c',
                  'log.c', 'logf.c', 'logl.c', 'pow.c', 'powf.c', 'powl.c'])
 
-    return build_libc(libname, files, ['-O2'])
+    return build_libc(libname, files, ['-O2', '-fno-builtin'])
 
   # libcxx
   def create_libcxx(libname):
@@ -307,7 +311,7 @@ def dlmalloc_name():
 
   def create_dlmalloc(out_name):
     o = in_temp(out_name)
-    cflags = ['-O2']
+    cflags = ['-O2', '-fno-builtin']
     if shared.Settings.USE_PTHREADS:
       cflags += ['-s', 'USE_PTHREADS=1']
     if shared.Settings.EMSCRIPTEN_TRACING:
@@ -340,7 +344,9 @@ def create_wasm_rt_lib(libname, files):
         '-mthread-model', 'single',
         output_flag,
         shared.path_from_root('system', 'lib', src),
-        '-O2', '-o', o] + musl_internal_includes() + shared.EMSDK_OPTS)
+        '-O2', '-fno-builtin', '-o', o] +
+        musl_internal_includes() +
+        shared.EMSDK_OPTS)
       o_s.append(o)
     run_commands(commands)
     lib = in_temp(libname)
