[Info-vax] LLVM, volatile and async VMS I/O and system calls

[Bill Gunshannon]

On 10/3/21 8:29 AM, chris wrote:
> On 10/03/21 10:21, Jan-Erik Söderholm wrote:
>> Den 2021-10-03 kl. 01:06, skrev chris:
>>>
>>> So, it really comes down to optimiser choice of instruction, depending
>>> on the use of the volatile keyword. Perhaps optimisers might optimise
>>> such reads out, but that common ?.
>>
>> Optimizing out code not needed is one of the most common things that
>> optimizers does. A very short example.
>>
>> int a;
>> int b;
>> void main() {
>> a = 10;
>> b = a;
>> b = a;
>> }
>>
>> The second assignment to b will simply be removed since it is
>> a duplicate of the first and the value of a has not changed, at
>> least not as far as the compiler can see.
>>
>> Compare with:
>>
>> int volatile a;
>> int b;
>> void main() {
>> a = 10;
>> b = a;
>> b = a;
>> }
>>
>> This tells the compiler that the value of a can change outside of
>> the current compile unit and both assignments will be left to be
>> done and the value of b could be different after each assignment.
>>
>> The first generates 7 Alpha instructions and the second 9 instructions.
> 
> Would be interesting to see instructions generated, so we can see what
> is happening. Instruction count alone doesn't say much.

More interesting than you might thnk.  Here's an x86 version.

	.text
	.globl	main
	.type	main, @function
main:
.LFB0:
	.cfi_startproc
	movl	$10, a(%rip)
	movl	$10, b(%rip)
	ret
	.cfi_endproc
.LFE0:
	.size	main, .-main
	.comm	b,4,4
	.comm	a,4,4
	.ident	"GCC: (Ubuntu 7.5.0-3ubuntu1~18.04) 7.5.0"
	.section	.note.GNU-stack,"", at progbits

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

	.text
	.globl	main
	.type	main, @function
main:
.LFB0:
	.cfi_startproc
	movl	$10, a(%rip)
	movl	a(%rip), %eax
	movl	a(%rip), %eax
	movl	%eax, b(%rip)
	ret
	.cfi_endproc
.LFE0:
	.size	main, .-main
	.comm	b,4,4
	.comm	a,4,4
	.ident	"GCC: (Ubuntu 7.5.0-3ubuntu1~18.04) 7.5.0"
	.section	.note.GNU-stack,"", at progbits

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

And the diff:

< 	movl	$10, b(%rip)
---
 > 	movl	a(%rip), %eax
 > 	movl	a(%rip), %eax
 > 	movl	%eax, b(%rip)


Most interesting.

bill