Cortex-M3 low level assert issue.

[Mike Franklin]

On Wednesday, 1 May 2019 at 21:55:09 UTC, Jack Applegame wrote:

> LDC compiles `assert(0)` to `_d_assert(file, line)` call and 
> puts file name (test.d) to section `.rodata.str1.1`:
>
> ```
> Contents of section .rodata.str1.1:
>  0000 74657374 2e466f6f 2e426172 00746573  test.Foo.Bar.tes
>  0010 742e466f 6f007465 73742e64 00        t.Foo.test.d.
> ```
>
> The problem is that this section also contains symbols 
> `test.Foo.Bar` and `test.Foo`. I don't know why these symbols 
> are needed, but they are definitely not used in the program. 
> And because they are in the same section as `test.d`, this 
> whole section goes into firmware.
>
> In my real application, such useless data occupy an 
> unacceptable amount of flash memory of the MCU. It would be 
> nice to get rid of them.

If you'd like to see a more complete cortex-m project, please see 
https://github.com/JinShil/stm32f42_discovery_demo

You can see how I compile with ldc2 at 
https://github.com/JinShil/stm32f42_discovery_demo/blob/2f6419de1c30179a85c07e0bf1e3ddd026ace93c/build.d#L78-L88

The technique I use in that project is to create my own minimal 
runtime: 
https://github.com/JinShil/stm32f42_discovery_demo/tree/master/source/runtime It just contains the runtime implementations that I need; nothing more.

If you want to have support for asserts, you'll have to implement 
the runtime hooks as shown here:  
https://github.com/JinShil/stm32f42_discovery_demo/blob/master/source/runtime/exception.d

Note that it's ok for your .o files be full of junk.  They can be 
stripped out at link time with the `--gc-sections` flag.  Just be 
sure you're compiling with `-ffunction-sections` and 
`-fdata-sections` for `--gc-sections` to work well.

Question for LDC developers:  Are the above compiler flags 
enabled by default?  How can one confirm?

You can start your minimal runtime project with just an empty 
`object.d`:

object.d
```
module object;
```

test.d
```
module test;

struct Foo {
     struct Bar {
         struct Baz {
         }
     }
}

void foo() {
     assert(0);
}
```

```
ldc2 -conf= -c -mtriple=thumb-none--eabi -mcpu=cortex-m3 test.d
arm-none-eabi-size test.o
    text    data     bss     dec     hex filename
      48       0       0      48      30 test.o
```

Compare that without the object.d.  When the compiler detects a 
local object.d file, it will use it instead of the runtime.  The 
`-conf=` tells the compiler not to use the default configuration 
that imports the runtime that comes bundled with the compiler.  
Compile with `-v` and compare with and without the `-conf=` flag 
to see the difference.

If this just prompts more questions, let them fly.

Mike