Avoid gcc 7.2.1 compiler issues in nlr_push() (#506)

Avoid gcc 7.2.1 compiler issues in nlr_push()

1 files changed, 7 insertions, 1 deletions

diff --git a/py/nlrthumb.c b/py/nlrthumb.c
index 6e7d71766..0616e59b0 100644
--- a/py/nlrthumb.c
+++ b/py/nlrthumb.c
@@ -74,7 +74,13 @@ __attribute__((naked)) unsigned int nlr_push(nlr_buf_t *nlr) {
 #else
     "b      nlr_push_tail       \n" // do the rest in C
 #endif
-    );
+    :                               // output operands
+    : "r" (nlr)                     // input operands
+    // Do not use r1, r2, r3 as temporary saving registers.
+    // gcc 7.2.1 started doing this, and r3 got clobbered in nlr_push_tail.
+    // See https://github.com/adafruit/circuitpython/issues/500 for details.
+    : "r1", "r2", "r3"              // clobbers
+                    );
 
     return 0; // needed to silence compiler warning
 }