Linux: Use -mfpmath=sse to remove differences between opt and debug

build/SConscript.main

Index: build/SConscript.main
diff --git a/build/SConscript.main b/build/SConscript.main
index 4289b43d76fecf0313953dc6a148eaf4702d3aec..6f448c7140b6c214f939e5fd5e777a75f11f4170 100644
--- a/build/SConscript.main
+++ b/build/SConscript.main
@@ -475,7 +475,29 @@ if not root_env.get('_GYP'):
   )
 linux_env.Append(
     ASFLAGS = ['-32'],
-    CCFLAGS = ['-m32', '-pthread', '-march=i686', '-fno-exceptions'],
+    CCFLAGS = ['-m32', '-pthread', '-march=pentium4', '-fno-exceptions',
+    # All floating-point computations on x87 happens in 80-bit precision.
+    # Because the C and C++ language standards allow the compiler to keep the
+    # floating-point values in higher precision than what's specified in the
+    # source and doing so is more efficient than constantly rounding up to
+    # 64-bit or 32-bit precision as specified in the source, the compiler,
+    # especially in the optimized mode, tries very hard to keep values in x87
+    # floating-point stack (in 80-bit precision) as long as possible. This has
+    # important side effects, that the real value used in computation may
+    # change depending on how the compiler did the optimization - that is, the
+    # value kept in 80-bit is different than the value rounded down to 64-bit
+    # or 32-bit. There are possible compiler options to make this behavior
+    # consistent (e.g. -ffloat-store would keep all floating-values in the
+    # memory, thus force them to be rounded to its original precision) but they
+    # have significant runtime performance penalty.
+    #
+    # -mfpmath=sse -msse2 makes the compiler use SSE instructions which keep
+    # floating-point values in SSE registers in its native precision (32-bit
+    # for single precision, and 64-bit for double precision values). This means
+    # the floating-point value used during computation does not change
+    # depending on how the compiler optimized the code, since the value is
+    # always kept in its specified precision.
+               '-msse2', '-mfpmath=sse'],
     # GCC will generate ident directives with the GCC version. Accumulate
     # these all up and you end up with ~80K repeated in a .comment section.
     CCFLAGS_OPTIMIZED = ['-fno-ident'],