Left shifts into the sign bit is a kind of overflow, and the standard chooses to treat left shifts of negative values the same way.
However, the -fwrapv option modifies the language to one where integers are defined as two's complement---which also defines entirely the behavior of shifts. Disable sanitization of left shifts when -fwrapv is in effect, using the same logic as instrument_si_overflow. The same change was proposed for LLVM at https://llvm.org/bugs/show_bug.cgi?id=25552. Bootstrapped/regtested x86_64-pc-linux-gnu. Ok for trunk, and for GCC 5 branch after 5.3 is released? Thanks, Paolo gcc: PR sanitizer/68418 * c-family/c-ubsan.c (ubsan_instrument_shift): Disable sanitization of left shifts for wrapping signed types as well. gcc/testsuite: PR sanitizer/68418 * gcc.dg/ubsan/c99-wrapv-shift-1.c, gcc.dg/ubsan/c99-wrapv-shift-2.c: New testcases. Index: c-family/c-ubsan.c =================================================================== --- c-family/c-ubsan.c (revision 231289) +++ c-family/c-ubsan.c (working copy) @@ -124,12 +124,17 @@ ubsan_instrument_shift (location_t loc, enum tree_ t = fold_convert_loc (loc, op1_utype, op1); t = fold_build2 (GT_EXPR, boolean_type_node, t, uprecm1); + /* If this is not a signed operation, don't perform overflow checks. + Also punt on bit-fields. */ + if (!INTEGRAL_TYPE_P (type0) + || TYPE_OVERFLOW_WRAPS (type0) + || GET_MODE_BITSIZE (TYPE_MODE (type0)) != TYPE_PRECISION (type0)) + ; + /* For signed x << y, in C99/C11, the following: (unsigned) x >> (uprecm1 - y) if non-zero, is undefined. */ - if (code == LSHIFT_EXPR - && !TYPE_UNSIGNED (type0) - && flag_isoc99) + else if (code == LSHIFT_EXPR && flag_isoc99 && cxx_dialect < cxx11) { tree x = fold_build2 (MINUS_EXPR, op1_utype, uprecm1, fold_convert (op1_utype, unshare_expr (op1))); @@ -142,9 +147,7 @@ ubsan_instrument_shift (location_t loc, enum tree_ /* For signed x << y, in C++11 and later, the following: x < 0 || ((unsigned) x >> (uprecm1 - y)) if > 1, is undefined. */ - if (code == LSHIFT_EXPR - && !TYPE_UNSIGNED (type0) - && (cxx_dialect >= cxx11)) + else if (code == LSHIFT_EXPR && cxx_dialect >= cxx11) { tree x = fold_build2 (MINUS_EXPR, op1_utype, uprecm1, fold_convert (op1_utype, unshare_expr (op1))); Index: testsuite/gcc.dg/ubsan/c99-wrapv-shift-1.c =================================================================== --- testsuite/gcc.dg/ubsan/c99-wrapv-shift-1.c (revision 0) +++ testsuite/gcc.dg/ubsan/c99-wrapv-shift-1.c (working copy) @@ -0,0 +1,9 @@ +/* { dg-do run } */ +/* { dg-options "-fsanitize=shift -fwrapv -w -std=c99" } */ + +int +main (void) +{ + int a = -42; + a << 1; +} Index: testsuite/gcc.dg/ubsan/c99-wrapv-shift-2.c =================================================================== --- testsuite/gcc.dg/ubsan/c99-wrapv-shift-2.c (revision 0) +++ testsuite/gcc.dg/ubsan/c99-wrapv-shift-2.c (working copy) @@ -0,0 +1,9 @@ +/* { dg-do run } */ +/* { dg-options "-fsanitize=shift -fwrapv -w -std=c99" } */ + +int +main (void) +{ + int a = 1; + a <<= 31; +}
