BUG: Fix signed char assumption in einsum label parsing

1 files changed, 22 insertions, 5 deletions

diff --git a/numpy/core/src/multiarray/einsum.c.src b/numpy/core/src/multiarray/einsum.c.src
index 98a26c322..9898d00fa 100644
--- a/numpy/core/src/multiarray/einsum.c.src
+++ b/numpy/core/src/multiarray/einsum.c.src
@@ -1917,6 +1917,10 @@ parse_operand_subscripts(char *subscripts, int length,
     /*
      * Find any labels duplicated for this operand, and turn them
      * into negative offets to the axis to merge with.
+     *
+     * In C, the char type may be signed or unsigned, but with
+     * twos complement arithmetic the char is ok either way here, and
+     * later where it matters the char is cast to a signed char.
      */
     for (idim = 0; idim  < ndim-1; ++idim) {
         char *next;
@@ -1928,7 +1932,7 @@ parse_operand_subscripts(char *subscripts, int length,
                                     ndim-idim-1);
             while (next != NULL) {
                 /* The offset from next to out_labels[idim] (negative) */
-                *next = (out_labels+idim)-next;
+                *next = (char)((out_labels+idim)-next);
                 /* Search for the next matching label */
                 next = (char *)memchr(next+1, label,
                                         out_labels+ndim-1-next);
@@ -2128,7 +2132,11 @@ get_single_op_view(PyArrayObject *op, int  iop, char *labels,
 
     /* Match the labels in the operand with the output labels */
     for (idim = 0; idim < ndim; ++idim) {
-        label = labels[idim];
+        /*
+         * The char type may be either signed or unsigned, we
+         * need it to be signed here.
+         */
+        label = (signed char)labels[idim];
         /* If this label says to merge axes, get the actual label */
         if (label < 0) {
             label = labels[idim+label];
@@ -2226,7 +2234,11 @@ get_combined_dims_view(PyArrayObject *op, int iop, char *labels)
     /* Copy the dimensions and strides, except when collapsing */
     icombine = 0;
     for (idim = 0; idim < ndim; ++idim) {
-        label = labels[idim];
+        /*
+         * The char type may be either signed or unsigned, we
+         * need it to be signed here.
+         */
+        label = (signed char)labels[idim];
         /* If this label says to merge axes, get the actual label */
         if (label < 0) {
             combineoffset = label;
@@ -2877,10 +2889,15 @@ PyArray_EinsteinSum(char *subscripts, npy_intp nop,
             }
         }
 
-        /* Check whether any dimensions need to be combined */
+        /*
+         * Check whether any dimensions need to be combined
+         *
+         * The char type may be either signed or unsigned, we
+         * need it to be signed here.
+         */
         combine = 0;
         for (idim = 0; idim < ndim; ++idim) {
-            if (labels[idim] < 0) {
+            if ((signed char)labels[idim] < 0) {
                 combine = 1;
             }
         }