Gnat for ARM : are these bugs?

Gnat for ARM : are these bugs?

[jan.de.kruyf]

Hallo,
I was doing some code patterns for a little Oberon compiler, as a diversion in my free time.
And 2 most extraorinary features popped up :

1. its seems to be illegal to to assign 2 ** 0 ( = 1) to a mod type.

2. on a illegal assignment like 2 ** 32 to a 32 bit mod type the answer is a big fat zero with no questions asked.

Does anybody have an explanation that I might have overlooked?

Thanks

j.

---
here is the relevant code and the assembler output from gcc

-- compiled with arm-eabi-gcc -S -mtune=cortex-m4 -mfloat-abi=hard patset.adb

procedure Patset is
   type Set is mod 2**32;
   N : Integer;             -- stored at stackpointer + 4
   S : Set;                 -- stored at stackpointer + 0
begin
   S := 2 ** N;
end Patset;

----------------------------------------------------------

_ada_patset:
	@ args = 0, pretend = 0, frame = 8
	@ frame_needed = 1, uses_anonymous_args = 0
	push	{r7, lr}                       -- procedure start
	sub	sp, sp, #8
	add	r7, sp, #0
	ldr	r3, [r7, #4]                   -- load 'N'
	cmp	r3, #0                         -- if 0 then exception
	bge	.L2                            -- else .L2
	movw	r0, #:lower16:.LC0             -- last chance handler set up
	movt	r0, #:upper16:.LC0
	mov	r1, #8
	bl	__gnat_last_chance_handler
.L2:
	ldr	r3, [r7, #4]                   -- load 'N'
	cmp	r3, #31                        -- if N > 31 then .L3
	bgt	.L3
	ldr	r3, [r7, #4]                   -- else normal processing
	mov	r2, #1
	lsl	r3, r2, r3
	b	.L4
.L3:
	mov	r3, #0                         -- on error my set becomes {}
.L4:
	str	r3, [r7, #0]
	Nop                                    -- procedure end game
	add	r7, r7, #8
	mov	sp, r7
	pop	{r7, pc}

--------------------------------------------------------------

Re: Gnat for ARM : are these bugs?

[jan.de.kruyf]

ok,

i was too much in a rush.
Reading the right books enlightened the issues.

Issue 1 is explained by correctly reading the assembly: BGE means signed 'equal or greater'.

issue 2 follows from the definition of a mod type. it will automatically roll over when the result of an operation is too big for the type to contain.

In any case: All, thanks for listening.

j.

Re: Gnat for ARM : are these bugs?

[Egil H H]

On Tuesday, May 19, 2015 at 9:54:32 AM UTC+2, jan.de...@gmail.com wrote:
> 
> 1. its seems to be illegal to to assign 2 ** 0 ( = 1) to a mod type.

Works fine for me (on x86, though).
No expert on ARM-assembly but I read bge as greater or equal, so:
	cmp	r3, #0                         -- if N<0 then exception
	bge	.L2                            -- else .L2

> 
> 2. on a illegal assignment like 2 ** 32 to a 32 bit mod type the answer is a big fat zero with no questions asked.

All operations on modular types are performed modulo the modulus, ** is no exception from that rule.
If you want it to fail, try instead to assign a named number:

   bar : constant := 2**N;
begin
   S := bar;

-- 
~egilhh