• visitor (11/11) Nov 23 2020 Hi all,
• Adam D. Ruppe (11/14) Nov 23 2020 The .ptr[0] skips bounds checking.
• visitor (4/19) Nov 23 2020 indeed because of the the static length and constant index, it

visitor <pierredavant gmail.com> writes:

Hi all,

I would like to know why in the code below, rgba.ptr[0] is used 
instead of rgba[0] and allowing the method to be  safe

float[4] rgba = 0;
ref inout(float) r() inout pure  trusted { pragma(inline, true); 
return rgba.ptr[0]; }

why not :
ref inout(float) r() inout pure  safe { pragma(inline, true); 
return rgba[0]; }

avoid an allocation maybe ?

Thanks for your time.

Adam D. Ruppe <destructionator gmail.com> writes:

On Monday, 23 November 2020 at 17:34:27 UTC, visitor wrote:
 Hi all,

 I would like to know why in the code below, rgba.ptr[0] is used 
 instead of rgba[0] and allowing the method to be  safe

The .ptr[0] skips bounds checking.

Since this example is static length with a constant index it 
shouldn't matter anyway; the compiler can see it is obviously in 
bounds and skip it too.

But if there's any runtime value there's a bounds check with 
`foo[0]` and that can be surprisingly expensive in certain 
situations. So `foo.ptr[0]` skipping that can give a nice 
performance boost.

Just without bounds checking the code is obviously trusting the 
programmer... hence  trusted is required instead of safe.

visitor <pierredavant gmail.com> writes:

On Monday, 23 November 2020 at 17:39:09 UTC, Adam D. Ruppe wrote:
 On Monday, 23 November 2020 at 17:34:27 UTC, visitor wrote:
 Hi all,

 I would like to know why in the code below, rgba.ptr[0] is 
 used instead of rgba[0] and allowing the method to be  safe

 The .ptr[0] skips bounds checking.

 Since this example is static length with a constant index it 
 shouldn't matter anyway; the compiler can see it is obviously 
 in bounds and skip it too.

 But if there's any runtime value there's a bounds check with 
 `foo[0]` and that can be surprisingly expensive in certain 
 situations. So `foo.ptr[0]` skipping that can give a nice 
 performance boost.

 Just without bounds checking the code is obviously trusting the 
 programmer... hence  trusted is required instead of safe.


indeed because of the the static length and constant index, it 
was puzzling me ...
Thanks Adam for clarification