Switch -g disables optimizations.

[Joakim via digitalmars-d-ldc]

On Friday, 9 October 2015 at 04:22:51 UTC, Marco Leise wrote:
> I found the root for the horrible codegen with ldc2 I was 
> experiencing. As soon as I added -g to get some line numbers in 
> disassemblies, one or more optimizations steps got disabled.
>
>   int main(string[])
>   {
>     return 0;
>   }
>
> compiles with -O to:
>
>   xor eax,eax
>   ret
>
> and with -O -g to:
>
>   push   rbp
>   mov    rbp,rsp
>   xor    eax,eax
>   pop    rbp
>   ret
>
> ldc2 -version:
> LDC - the LLVM D compiler (0.16.0):
>   based on DMD v2.067.1 and LLVM 3.6.2
>
> Is that expected to improve the debugging experience and make 
> local variables always available or a bug? clang's codegen is 
> not influenced in this way by the -g flag.

Optimization is turned off in debug mode, with a comment claiming 
debug info doesn't work with optimization:

https://github.com/ldc-developers/ldc/blob/9d3dd1f609e08f001aeec2cacb1c55707f37884b/gen/optimizer.cpp#L179