I'd like to backport the following patch that suppresses bogus ubsan errors. I had to tweak the testcase a bit since 4.9 doesn't know -fno-sanitize-recover.
Bootstrapped/regtested on x86_64-linux, ok for 4.9? 2015-03-10 Marek Polacek <pola...@redhat.com> Backported from mainline 2014-12-04 Marek Polacek <pola...@redhat.com> PR middle-end/56917 * fold-const.c (fold_unary_loc): Perform the negation in A's type when transforming ~ (A - 1) or ~ (A + -1) to -A. * c-c++-common/ubsan/pr56917.c: New test. --- gcc/fold-const.c +++ gcc/fold-const.c @@ -8324,9 +8324,14 @@ fold_unary_loc (location_t loc, enum tree_code code, tree type, tree op0) && integer_onep (TREE_OPERAND (arg0, 1))) || (TREE_CODE (arg0) == PLUS_EXPR && integer_all_onesp (TREE_OPERAND (arg0, 1))))) - return fold_build1_loc (loc, NEGATE_EXPR, type, - fold_convert_loc (loc, type, - TREE_OPERAND (arg0, 0))); + { + /* Perform the negation in ARG0's type and only then convert + to TYPE as to avoid introducing undefined behavior. */ + tree t = fold_build1_loc (loc, NEGATE_EXPR, + TREE_TYPE (TREE_OPERAND (arg0, 0)), + TREE_OPERAND (arg0, 0)); + return fold_convert_loc (loc, type, t); + } /* Convert ~(X ^ Y) to ~X ^ Y or X ^ ~Y if ~X or ~Y simplify. */ else if (TREE_CODE (arg0) == BIT_XOR_EXPR && (tem = fold_unary_loc (loc, BIT_NOT_EXPR, type, --- gcc/testsuite/c-c++-common/ubsan/pr56917.c +++ gcc/testsuite/c-c++-common/ubsan/pr56917.c @@ -0,0 +1,43 @@ +/* PR middle-end/56917 */ +/* { dg-do run } */ +/* { dg-options "-fsanitize=undefined" } */ + +#include <stdio.h> + +#define INT_MIN (-__INT_MAX__ - 1) +#define LONG_MIN (-__LONG_MAX__ - 1L) +#define LLONG_MIN (-__LONG_LONG_MAX__ - 1LL) + +int __attribute__ ((noinline,noclone)) +fn1 (unsigned int u) +{ + return (-(int) (u - 1U)) - 1; +} + +long __attribute__ ((noinline,noclone)) +fn2 (unsigned long int ul) +{ + return (-(long) (ul - 1UL)) - 1L; +} + +long long __attribute__ ((noinline,noclone)) +fn3 (unsigned long long int ull) +{ + return (-(long long) (ull - 1ULL)) - 1LL; +} + +int +main (void) +{ + fputs ("UBSAN TEST START\n", stderr); + + if (fn1 (__INT_MAX__ + 1U) != INT_MIN + || fn2 (__LONG_MAX__ + 1UL) != LONG_MIN + || fn3 (__LONG_LONG_MAX__ + 1ULL) != LLONG_MIN) + __builtin_abort (); + + fputs ("UBSAN TEST END\n", stderr); + return 0; +} + +/* { dg-output "UBSAN TEST START(\n|\r\n|\r)UBSAN TEST END" } */ Marek