PR26447 UBSAN: expr.c:1936 left shift of negative value
Alan Modra
amodra@gmail.com
Wed Aug 26 13:47:10 GMT 2020
PR 26447
* expr.c (expr <O_left_shift>): Do an unsigned shift.
diff --git a/gas/expr.c b/gas/expr.c
index 23a6a003b9..7c002c0d96 100644
--- a/gas/expr.c
+++ b/gas/expr.c
@@ -1933,12 +1933,21 @@ expr (int rankarg, /* Larger # is higher rank. */
case O_multiply: resultP->X_add_number *= v; break;
case O_divide: resultP->X_add_number /= v; break;
case O_modulus: resultP->X_add_number %= v; break;
- case O_left_shift: resultP->X_add_number <<= v; break;
+ case O_left_shift:
+ /* We always use unsigned shifts. According to the ISO
+ C standard, left shift of a signed type having a
+ negative value is undefined behaviour, and right
+ shift of a signed type having negative value is
+ implementation defined. Left shift of a signed type
+ when the result overflows is also undefined
+ behaviour. So don't trigger ubsan warnings or rely
+ on characteristics of the compiler. */
+ resultP->X_add_number
+ = (valueT) resultP->X_add_number << (valueT) v;
+ break;
case O_right_shift:
- /* We always use unsigned shifts, to avoid relying on
- characteristics of the compiler used to compile gas. */
- resultP->X_add_number =
- (offsetT) ((valueT) resultP->X_add_number >> (valueT) v);
+ resultP->X_add_number
+ = (valueT) resultP->X_add_number >> (valueT) v;
break;
case O_bit_inclusive_or: resultP->X_add_number |= v; break;
case O_bit_or_not: resultP->X_add_number |= ~v; break;
--
Alan Modra
Australia Development Lab, IBM
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