Re: rgba.ptr[0] vs rgba[0]

[visitor via Digitalmars-d-learn]

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

Re: rgba.ptr[0] vs rgba[0]

[Adam D. Ruppe via Digitalmars-d-learn]

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.

rgba.ptr[0] vs rgba[0]

[visitor via Digitalmars-d-learn]

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.