asm volatile statement reordering

[Segher Boessenkool]

Hi!

On Tue, Oct 17, 2017 at 12:27:08AM +0200, David Brown wrote:
> There has been a discussion going on about asm volatile statement 
> reordering, where there has been surprising code generated for the ARM 
> with certain code and compiler flags.  The discussion has been in the 
> comp.arch.embedded Usenet group, the gcc-arm-embedded project (the most 
> common source of gcc for embedded ARM devices, both for individuals and 
> for companies), and some other related projects.  I believe it is time 
> to ask the gcc folks too!
> 
> This is a link to the gcc-arm-embedded issue, which is perhaps the most 
> complete version.
> 
> <https://bugs.launchpad.net/gcc-arm-embedded/+bug/1722849?comments=all>

I cannot reproduce this problem (with trunk GCC).  What is different
about the compiler used there / what else is different?

...

I now also tried with some GCC 5, and the problem does happen there.

Please open a PR!

This is similar to PR62642, which is about unspec_volatile -- essentially
everything that applies to unspec_volatile also applies to volatile asm.

I did the following patch, which fixes the problem in the testcase.
Please try it with the "real" code?  (And do open a PR please).


> Much of this boils down to the question of when gcc is allowed to 
> re-order "asm volatile" statements, with respect to other "asm volatile" 
> statements, volatile memory accesses, and unknown functions (which may 
> contain observable behaviour).

"asm volatile" means the asm has an (unknown) side effect when executed.
It cannot be moved over any other side effects.

> My testing suggests that gcc will re-order "asm volatile" statements 
> that have an output, such as the "save the PRIMASK into status" 
> statement, but it will /not/ re-order "asm volatile" statements that 
> have no outputs.
> 
> Is that correct?

Nope, you found a bug :-)


Segher


Index: gcc/ira.c
===================================================================
--- gcc/ira.c   (revision 251105)
+++ gcc/ira.c   (working copy)
@@ -4418,6 +4418,10 @@
 	 for a reason.  */
       return false;
 
+    case ASM_OPERANDS:
+      if (MEM_VOLATILE_P (x))
+	return false;
+
     default:
       break;
     }