decodetree: Properly diagnose fields overflowing an insn

Previously this would result in an exception for shifting
the field mask by a negative number.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

scripts/decodetree.py

@@ -622,6 +622,8 @@ def parse_generic(lineno, is_format, name, toks):
                 sign = True
                 flen = flen[1:]
             shift = int(flen, 10)
+            if shift + width > insnwidth:
+                error(lineno, 'field {0} exceeds insnwidth'.format(fname))
             f = Field(sign, insnwidth - width - shift, shift)
             flds = add_field(lineno, flds, fname, f)
             fixedbits <<= shift

tests/decode/err_width1.decode

@@ -0,0 +1,5 @@
+# This work is licensed under the terms of the GNU LGPL, version 2 or later.
+# See the COPYING.LIB file in the top-level directory.
+
+# Diagnose too many bits (33 of 32)
+one	000000000000000000000000000000000

tests/decode/err_width2.decode

@@ -0,0 +1,5 @@
+# This work is licensed under the terms of the GNU LGPL, version 2 or later.
+# See the COPYING.LIB file in the top-level directory.
+
+# Diagnose too few bits (31 of 32)
+one	0000000000000000000000000000000

tests/decode/err_width3.decode

@@ -0,0 +1,5 @@
+# This work is licensed under the terms of the GNU LGPL, version 2 or later.
+# See the COPYING.LIB file in the top-level directory.
+
+# Diagnose too many bits (33 of 32)
+one	0 s:32

tests/decode/err_width4.decode

@@ -0,0 +1,5 @@
+# This work is licensed under the terms of the GNU LGPL, version 2 or later.
+# See the COPYING.LIB file in the top-level directory.
+
+# Diagnose too few bits (31 of 32)
+one	0 s:30