runtime: fix arm preemption

Preemption during the software floating point code
could cause m (R9) to change, so that when the
original registers were restored at the end of the
floating point handler, the changed and correct m
would be replaced by the old and incorrect m.

TBR=dvyukov
CC=golang-dev
https://golang.org/cl/11883045

diff --git a/src/pkg/runtime/vlop_arm.s b/src/pkg/runtime/vlop_arm.s
index 02eb985f7b91fd63db030f5e13c6439452c6b48f..cd56898dc6da0cd95344376b829f72263aef4d41 100644 (file)
--- a/src/pkg/runtime/vlop_arm.s
+++ b/src/pkg/runtime/vlop_arm.s
@@ -59,7 +59,15 @@ TEXT _sfloat(SB), 7, $64-0 // 4 arg + 14*4 saved regs + cpsr
        MOVW    64(R13), R1
        WORD    $0xe128f001     // msr cpsr_f, r1
        MOVW    $12(R13), R0
-       MOVM.IA.W       (R0), [R1-R12]
+       // Restore R1-R8 and R11-R12, but ignore the saved R9 (m) and R10 (g).
+       // Both are maintained by the runtime and always have correct values,
+       // so there is no need to restore old values here.
+       // The g should not have changed, but m may have, if we were preempted
+       // and restarted on a different thread, in which case restoring the old
+       // value is incorrect and will cause serious confusion in the runtime.
+       MOVM.IA.W       (R0), [R1-R8]
+       MOVW    $52(R13), R0
+       MOVM.IA.W       (R0), [R11-R12]
        MOVW    8(R13), R0
        RET