Re: [Sablevm-developer] Porting SableVM to ARM: Questions

[Archie C. <ar...@de...>]

Gunda Domlur wrote:
> Now I am back after the holidays to look at the problem of making the 
> inlined version working on ARM.

I happen to have been working with ARM assembly lately, on an
unrelated project. One thing that may present difficulties for
SableVM inline threading is the way that immediate values are
handled.

The ARM instruction set does not allow loading of arbitrary
immediate values; to typically such values are stored in a
'literals' pool at the end of whatever function, etc. you are
assembling or compiling, and then they are loaded using a
PC-relative instruction.

This means that if you move the code, but don't move the
literals, it won't work. Moreover, it's hard to predict when
the compiler is going to do this, because some constants
can be loaded as immediate values (specifically, those that
can be represented as an 8-bit value shifted an even number
of bits, or the ones complement of such a value).

Here's an example. Consider this C code:

	extern int x;
	extern int y;
	void
	foo()
	{
		x = 0xff000000;	/* can be loaded as immediate */
		y = 0x12345678;	/* cannot be loaded as immediate */
	}

Here's what GCC outputs:

		.file   "foo.c"
		.text
		.align  2
		.global foo
		.type   foo,function
	foo:
		@ args = 0, pretend = 0, frame = 0
		@ frame_needed = 0, uses_anonymous_args = 0
		@ link register save eliminated.
		ldr     r3, .L2
		mov     r2, #-16777216
		ldr     r1, .L2+4
		str     r2, [r3, #0]
		ldr     r3, .L2+8
		@ lr needed for prologue
		str     r1, [r3, #0]
		bx      lr
	.L3:
		.align  2
	.L2:
		.word   x
		.word   305419896
		.word   y
	.Lfe1:
		.size   foo,.Lfe1-foo
		.ident  "GCC: (GNU) 3.2"


Note that the 0x12345678 (305419896) constant is stored in the literal
pool (starting at .L2) but the 0xff000000 (-16777216) constant is not.
The latter is 'relocatable' but the former is not.

So this is going to be tricky to predict which instructions are
inlinable; it also depends on how the compiler compiles code.

-Archie

__________________________________________________________________________
Archie Cobbs     *     Packet Design     *     http://www.packetdesign.com