> I have run into a strange issue on alpha that I'm still tracking down.
> I fear this has interrupted me too long to get 5.22 in for OpenBSD 5.9,
> but maybe we can get ahead of the curve and be ready after unlock.
>
> Previously, NaN + 1 looked like this:
> $ perl -we 'print "NaN" + 1'
> -nan
>
> Due to improvements in the Inf/NaN code, 5.22 should get:
> $ perl -we 'print "NaN" + 1'
> NaN
>
> But for some reason on alpha NaN isn't special and we instead get:
> $ ./perl -we 'print "NaN" + 1'
> 1
You might want to try this compiler diff on alpha.
When compiling with optimization enabled and ieee-style floating point, the
compiler tries to insert asynchronous fpu trap synchronization barriers as
late as possible.
Unfortunately, the logic does not take into account the store of a
floating-point result into memory as something requiring a barrier, which
leads to incorrect behaviour on alpha processors without the ``precise
arithmetic trap'' extension.
Index: alpha.c
===================================================================
RCS file: /OpenBSD/src/gnu/gcc/gcc/config/alpha/alpha.c,v
retrieving revision 1.4
diff -u -p -r1.4 alpha.c
--- alpha.c 20 Dec 2012 13:58:06 -0000 1.4
+++ alpha.c 17 Jan 2016 19:42:44 -0000
@@ -8721,11 +8721,15 @@ summarize_insn (rtx x, struct shadow_sum
result of an instruction that might generate an UNPREDICTABLE
result.
- (c) Within the trap shadow, no register may be used more than once
+ (c) Within the trap shadow, the destination register of the potentially
+ trapping instruction may not be used as an input, for its value would be
+ UNPREDICTABLE.
+
+ (d) Within the trap shadow, no register may be used more than once
as a destination register. (This is to make life easier for the
trap-handler.)
- (d) The trap shadow may not include any branch instructions. */
+ (e) The trap shadow may not include any branch instructions. */
static void
alpha_handle_trap_shadows (void)
@@ -8797,7 +8801,7 @@ alpha_handle_trap_shadows (void)
if ((sum.defd.i & shadow.defd.i)
|| (sum.defd.fp & shadow.defd.fp))
{
- /* (c) would be violated */
+ /* (d) would be violated */
goto close_shadow;
}
@@ -8820,11 +8824,19 @@ alpha_handle_trap_shadows (void)
goto close_shadow;
}
+
+ if ((sum.used.i & shadow.defd.i)
+ || (sum.used.fp & shadow.defd.fp))
+ {
+ /* (c) would be violated */
+ goto close_shadow;
+ }
break;
case JUMP_INSN:
case CALL_INSN:
case CODE_LABEL:
+ /* (e) would be violated */
goto close_shadow;
default:
