build: disable unsafe optimizations

Compile at -O2 and disable optimizations that trigger gcc bugs.

Some people still reported mksnapshot crashes after commit b40f813 ("build: fix
spurious mksnapshot crashes for good" - so much for that).

Average performance of the -O2 binary is on par with the -O3 binary. Variance
on the http_simple bytes/8 benchmark appears to be slightly greater but small
enough that the possibly of it being noise cannot be excluded.

The new binary very slightly but consistently outperforms the -O3 binary (by
about 0.5%) on the mostly CPU-bound bytes/102400 benchmark. That could be an
artifact of the system I benchmarked it on, a Core 2 Duo with a puny 32 kB of
L1 instruction cache. The smaller binary seems to play nicer with the cache.

common.gypi

@@ -8,6 +8,10 @@
     'component%': 'static_library',  # NB. these names match with what V8 expects
     'msvs_multi_core_compile': '0',  # we do enable multicore compiles, but not using the V8 way
 
+    # Turn on optimizations that may trigger compiler bugs.
+    # Use at your own risk. Do *NOT* report bugs if this option is enabled.
+    'node_unsafe_optimizations%': 0,
+
     # Enable V8's post-mortem debugging only on unix flavors.
     'conditions': [
       ['OS != "win"', {
@@ -41,13 +45,17 @@
         },
       },
       'Release': {
-        # Do *NOT* enable -ffunction-sections or -fdata-sections again.
-        # We don't link with -Wl,--gc-sections so they're effectively no-ops.
-        # Worse, they trigger very nasty bugs in some versions of gcc, notably
-        # v4.4.6 on x86_64-redhat-linux (i.e. RHEL and CentOS).
-        'cflags!': [ '-ffunction-sections', '-fdata-sections' ],
-        'cflags': [ '-O3' ],
         'conditions': [
+          ['node_unsafe_optimizations==1', {
+            'cflags': [ '-O3', '-ffunction-sections', '-fdata-sections' ],
+            'ldflags': [ '-Wl,--gc-sections' ],
+          }, {
+            'cflags': [ '-O2', '-fno-strict-aliasing', '-fno-tree-vrp' ],
+            'cflags!': [ '-O3',
+                         '-fstrict-aliasing',
+                         '-ffunction-sections',
+                         '-fdata-sections' ],
+          }],
           ['target_arch=="x64"', {
             'msvs_configuration_platform': 'x64',
           }],

configure

@@ -297,7 +297,6 @@ def configure_node(o):
   # SunOS, and we haven't implemented it.)
   if sys.platform.startswith('sunos'):
     o['variables']['node_use_dtrace'] = b(not options.without_dtrace)
-    o['variables']['v8_no_strict_aliasing'] = 1 # work around compiler bug
   elif b(options.with_dtrace) == 'true':
     raise Exception('DTrace is currently only supported on SunOS systems.')
   else:

deps/v8/build/common.gypi

@@ -332,10 +332,6 @@
         'conditions': [
           ['OS=="linux" or OS=="freebsd" or OS=="openbsd" or OS=="netbsd" \
             or OS=="android"', {
-            'cflags!': [
-              '-O2',
-              '-Os',
-            ],
             'cflags': [
               '-fdata-sections',
               '-ffunction-sections',