LDC, ARM: unnecessary default initialization
I explore the possibility of using D for bare metal ARM
programming.
For some reason, the LDC default initializes the structure, even
if initialization of all its members is specified as void. I
believe that this is wrong.
test.d
module test;
import core.bitop : volatileStore;
struct Foo {
uint[64] m = void; // no default initialization
this(uint a) {
foreach(ref b; m) volatileStore(&b,a++);
}
}
void ResetHandler() {
auto foo = Foo(10);
}
$ldc2 -mtriple=thumb-none-linux-eabi -mcpu=cortex-m3 -c --betterC
--boundscheck=off -relocation-model=static -O3 -vcolumns test.d
test.o assembly
<_D4test12ResetHandlerFZv>:
0: b510push{r4, lr}
2: b0c0sub sp, #256; 0x100 -+
4: 466cmov r4, sp |
default initialization
6: f44f 7180 mov.w r1, #256; 0x100 |
a: 4620mov r0, r4 | WHY???
c: f7ff fffe bl 0 <__aeabi_memclr4> -+
10: 2000movsr0, #0
12: f100 010a add.w r1, r0, #10
16: f844 1020 str.w r1, [r4, r0, lsl #2]
1a: 3001addsr0, #1
1c: 2949cmp r1, #73 ; 0x49
1e: d1f8bne.n 12 <_D4test12ResetHandlerFZv+0x12>
20: b040add sp, #256; 0x100
22: bd10pop {r4, pc}
*
Re: LDC, ARM: unnecessary default initialization
On Friday, 18 August 2017 at 09:42:25 UTC, Jack Applegame wrote: For some reason, the LDC default initializes the structure, even if initialization of all its members is specified as void. I believe that this is wrong. Afaik, this has been brought up multiple times already and is so by design. Every aggregate has an init symbol, omitting that (and accordingly the default initialization of all instances) by initializing each field with void doesn't work. The initialization isn't performed fieldwise, but is a bitcopy of T.init. You can skip initialization of specific instances though - `S s = void;` - but again not if `s` is a field of another aggregate.
Re: LDC, ARM: unnecessary default initialization
On Friday, 18 August 2017 at 12:09:04 UTC, kinke wrote:
On Friday, 18 August 2017 at 09:42:25 UTC, Jack Applegame wrote:
For some reason, the LDC default initializes the structure,
even if initialization of all its members is specified as
void. I believe that this is wrong.
Afaik, this has been brought up multiple times already and is
so by design. Every aggregate has an init symbol, omitting that
(and accordingly the default initialization of all instances)
by initializing each field with void doesn't work. The
initialization isn't performed fieldwise, but is a bitcopy of
T.init.
You can skip initialization of specific instances though - `S s
= void;` - but again not if `s` is a field of another aggregate.
Sorry, I forgot some workaround code:
void ResetHandler() {
Foo foo = void;
foo.__ctor(10);
// or: std.conv.emplace(&foo, 10);
}
Re: LDC, ARM: unnecessary default initialization
On Friday, 18 August 2017 at 17:28:38 UTC, kinke wrote:
On Friday, 18 August 2017 at 12:09:04 UTC, kinke wrote:
On Friday, 18 August 2017 at 09:42:25 UTC, Jack Applegame
wrote:
For some reason, the LDC default initializes the structure,
even if initialization of all its members is specified as
void. I believe that this is wrong.
Afaik, this has been brought up multiple times already and is
so by design. Every aggregate has an init symbol, omitting
that (and accordingly the default initialization of all
instances) by initializing each field with void doesn't work.
The initialization isn't performed fieldwise, but is a bitcopy
of T.init.
You can skip initialization of specific instances though - `S
s = void;` - but again not if `s` is a field of another
aggregate.
Sorry, I forgot some workaround code:
void ResetHandler() {
Foo foo = void;
foo.__ctor(10);
// or: std.conv.emplace(&foo, 10);
}
Thanks for the answer. Also I found another workaround code:
test.d
module test;
import core.bitop : volatileStore;
struct Foo {
uint[64] m = void; // no default initialization
static auto opCall(uint a) {
Foo foo = void;
foreach(ref b; foo.m) volatileStore(&b,a++);
return foo;
}
}
void ResetHandler() {
auto foo = Foo(10);
}
assembly
<_D4test12ResetHandlerFZv>:
0: b0c0sub sp, #256; 0x100
2: 2000movsr0, #0
4: 4669mov r1, sp
6: f100 020a add.w r2, r0, #10
a: f841 2020 str.w r2, [r1, r0, lsl #2]
e: 3001addsr0, #1
10: 2a49cmp r2, #73 ; 0x49
12: d1f8bne.n 6 <_D4test12ResetHandlerFZv+0x6>
14: b040add sp, #256; 0x100
16: 4770bx lr
Re: LDC, ARM: unnecessary default initialization
On Friday, 18 August 2017 at 12:09:04 UTC, kinke wrote: On Friday, 18 August 2017 at 09:42:25 UTC, Jack Applegame wrote: For some reason, the LDC default initializes the structure, even if initialization of all its members is specified as void. I believe that this is wrong. Afaik, this has been brought up multiple times already and is so by design. https://issues.dlang.org/show_bug.cgi?id=11331 https://issues.dlang.org/show_bug.cgi?id=11817 https://issues.dlang.org/show_bug.cgi?id=15951 It was discussed briefly during my DConf 2016 talk, and the status is that it is considered a performance bug. But it'll require spec changes / clarifications; Johannes listed a number already and maybe more it needed. - Johan
Re: LDC, ARM: unnecessary default initialization
On Tuesday, 22 August 2017 at 18:26:46 UTC, Johan Engelen wrote: It was discussed briefly during my DConf 2016 talk 2017.
