nothrow by default

[Gregor Mückl]

On Friday, 10 January 2020 at 07:06:31 UTC, Walter Bright wrote:
> On 1/9/2020 7:42 PM, Gregor Mückl wrote:
>> What exactly is the execution overhead for non-throwing code 
>> paths?
>> 
>> I believe I understand the overhead once stack unwinding needs 
>> to be performed, but how is code generation affected for the 
>> normal path?
>
> You need a mechanism for jumping to the scope guard code. Can't 
> do any code motion optimizations across the boundaries of such 
> code. Can't do register assignments for variables that are 
> "live" across that boundary.
>
> Just write some code with destructors in C++, compile it with 
> and without exceptions enabled, and dump the generated 
> assembler.

I played around with this as you suggested. The most concise 
example that I could find where -fno-exceptions makes a 
difference is this:

https://godbolt.org/z/WxnsOm

The version with exceptions reserves a register for something at 
the start of the function (rbp in this case), but I can't tell 
what it's for. It is used in the unwinding handler that is 
appended to the function after the ret instruction.