Avoid undefined behaviour when overshifting

Shifting by a negative amount or more than the width of the value is
undefined behaviour in C.  Adopt the semantics from Perl 5:

 - Negative shift inverts the direction
 - Left overshift yields zero
 - Right overshift yields zero for positive numbers,
   -1 for negative numbers

build/Makefile.in

@@ -80,6 +80,7 @@ OBJECTS = src/core/callsite@obj@ \
           src/core/ops@obj@ \
           src/core/hll@obj@ \
           src/core/loadbytecode@obj@ \
+          src/math/int@obj@ \
           src/math/num@obj@ \
           src/core/coerce@obj@ \
           src/core/dll@obj@ \

src/core/interp.c

@@ -745,11 +745,11 @@ void MVM_interp_run(MVMThreadContext *tc, void (*initial_invoke)(MVMThreadContex
                 cur_op += 4;
                 goto NEXT;
             OP(blshift_i):
-                GET_REG(cur_op, 0).i64 = GET_REG(cur_op, 2).i64 << GET_REG(cur_op, 4).i64;
+                GET_REG(cur_op, 0).i64 = MVM_int_shl(tc, GET_REG(cur_op, 2).i64, GET_REG(cur_op, 4).i64);
                 cur_op += 6;
                 goto NEXT;
             OP(brshift_i):
-                GET_REG(cur_op, 0).i64 = GET_REG(cur_op, 2).i64 >> GET_REG(cur_op, 4).i64;
+                GET_REG(cur_op, 0).i64 = MVM_int_shr(tc, GET_REG(cur_op, 2).i64, GET_REG(cur_op, 4).i64);
                 cur_op += 6;
                 goto NEXT;
             OP(pow_i): {

src/math/int.c

@@ -0,0 +1,21 @@
+#include "moar.h"
+
+/* copied from perl5/pp.c:S_iv_shift */
+static MVMint64 S_int_shift(MVMint64 value, MVMint64 shift, MVMint64 left) {
+   if (shift < 0) {
+       shift = -shift;
+       left = !left;
+   }
+   if (shift >= sizeof(MVMint64) * CHAR_BIT) {
+       return value < 0 && !left ? -1 : 0;
+   }
+   return left ? value << shift : value >> shift;
+}
+
+MVMint64 MVM_int_shl(MVMThreadContext *tc, MVMint64 value, MVMint64 shift) {
+    return S_int_shift(value, shift, 1);
+}
+
+MVMint64 MVM_int_shr(MVMThreadContext *tc, MVMint64 value, MVMint64 shift) {
+    return S_int_shift(value, shift, 0);
+}

src/math/int.h

@@ -0,0 +1,2 @@
+MVMint64 MVM_int_shl(MVMThreadContext *tc, MVMint64 value, MVMint64 shift);
+MVMint64 MVM_int_shr(MVMThreadContext *tc, MVMint64 value, MVMint64 shift);

src/moar.h

@@ -105,6 +105,7 @@ MVM_PUBLIC const MVMint32 MVM_jit_support(void);
 #include "core/threads.h"
 #include "core/hll.h"
 #include "core/loadbytecode.h"
+#include "math/int.h"
 #include "math/num.h"
 #include "core/coerce.h"
 #include "core/ext.h"