Enabling strict aliasing warnings in g++

Question

What is the correct way to enable strict aliasing warnings in g++? Does VC++ 10 implement those rules?

Answer 1

Use -fstrict-aliasing for g++. I also use -Wstrict-aliasing=2 to see warnings related to possible violations of strict aliasing rules.

Answer 2

They're enabled automatically by -O2 because it needs to use them to do some of the optimizations. Definitely combine it with the warning (-Wall does the trick) to make sure you aren't building potentially buggy code. Otherwise you can use -fstrict-aliasing as seen in another answer to enable them.

I'm not sure about VC++10 however.

Answer 3

VC++ 10 enables the strict aliasing rule with /O1 and above. I use the test program(with count value 6) in chapter 'BENEFITS TO THE STRICT ALIASING RULE' of here. And get following asm code. You can see the load of b is done only once.

00A910AE  movzx       edx,word ptr [edx+2]  //Load of b
00A910B2  xor         eax,eax  
00A910B4  xor         ecx,ecx  
00A910B6  add         dword ptr [esp+eax*4+34h],edx  //Loop start
00A910BA  add         eax,1  
00A910BD  adc         ecx,edi  
00A910BF  jne         main+76h (0A910C6h)  
00A910C1  cmp         eax,6  
00A910C4  jb          main+66h (0A910B6h)  //Loop end

But looks like there isn't a way to enable the warning for breaking this rule.