Richard Biener via Gcc-patches <gcc-patches@gcc.gnu.org> writes:
> The following makes sure to limit the shift operand when vectorizing
> (short)((int)x >> 31) via (short)x >> 31 as the out of bounds shift
> operand otherwise invokes undefined behavior. When we determine
> whether we can demote the operand we know we at most shift in the
> sign bit so we can adjust the shift amount.
>
> Note this has the possibility of un-CSEing common shift operands
> as there's no good way to share pattern stmts between patterns.
> We'd have to separately pattern recognize the definition.
>
> Bootstrapped on x86_64-unknown-linux-gnu, testing in progress.
>
> Not sure about LSHIFT_EXPR, it probably has the same issue but
> the fallback optimistic zero for out-of-range shifts is at least
> "corrrect". Not sure we ever try to demote rotates (probably not).
I guess you mean "correct" for x86? But that's just a quirk of x86.
IMO the behaviour is equally wrong for LSHIFT_EXPR.
Richard
> OK?
>
> Thanks,
> Richard.
>
> PR tree-optimization/110838
> * tree-vect-patterns.cc (vect_recog_over_widening_pattern):
> Adjust the shift operand of RSHIFT_EXPRs.
>
> * gcc.dg/torture/pr110838.c: New testcase.
> ---
> gcc/testsuite/gcc.dg/torture/pr110838.c | 43 +++++++++++++++++++++++++
> gcc/tree-vect-patterns.cc | 24 ++++++++++++++
> 2 files changed, 67 insertions(+)
> create mode 100644 gcc/testsuite/gcc.dg/torture/pr110838.c
>
> diff --git a/gcc/testsuite/gcc.dg/torture/pr110838.c
> b/gcc/testsuite/gcc.dg/torture/pr110838.c
> new file mode 100644
> index 00000000000..f039bd6c8ea
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/torture/pr110838.c
> @@ -0,0 +1,43 @@
> +/* { dg-do run } */
> +
> +typedef __UINT32_TYPE__ uint32_t;
> +typedef __UINT8_TYPE__ uint8_t;
> +typedef __INT8_TYPE__ int8_t;
> +typedef uint8_t pixel;
> +
> +/* get the sign of input variable (TODO: this is a dup, make common) */
> +static inline int8_t signOf(int x)
> +{
> + return (x >> 31) | ((int)((((uint32_t)-x)) >> 31));
> +}
> +
> +__attribute__((noipa))
> +static void calSign_bug(int8_t *dst, const pixel *src1, const pixel *src2,
> const int endX)
> +{
> + for (int x = 0; x < endX; x++)
> + dst[x] = signOf(src1[x] - src2[x]);
> +}
> +
> +__attribute__((noipa, optimize(0)))
> +static void calSign_ok(int8_t *dst, const pixel *src1, const pixel *src2,
> const int endX)
> +{
> + for (int x = 0; x < endX; x++)
> + dst[x] = signOf(src1[x] - src2[x]);
> +}
> +
> +__attribute__((noipa, optimize(0)))
> +int main()
> +{
> + const pixel s1[9] = { 0xcd, 0x33, 0xd4, 0x3e, 0xb0, 0xfb, 0x95, 0x64,
> 0x70, };
> + const pixel s2[9] = { 0xba, 0x9f, 0xab, 0xa1, 0x3b, 0x29, 0xb1, 0xbd,
> 0x64, };
> + int endX = 9;
> + int8_t dst[9];
> + int8_t dst_ok[9];
> +
> + calSign_bug(dst, s1, s2, endX);
> + calSign_ok(dst_ok, s1, s2, endX);
> +
> + if (__builtin_memcmp(dst, dst_ok, endX) != 0)
> + __builtin_abort ();
> + return 0;
> +}
> diff --git a/gcc/tree-vect-patterns.cc b/gcc/tree-vect-patterns.cc
> index ef806e2346e..e4ab8c2d65b 100644
> --- a/gcc/tree-vect-patterns.cc
> +++ b/gcc/tree-vect-patterns.cc
> @@ -3099,9 +3099,33 @@ vect_recog_over_widening_pattern (vec_info *vinfo,
> tree ops[3] = {};
> for (unsigned int i = 1; i < first_op; ++i)
> ops[i - 1] = gimple_op (last_stmt, i);
> + /* For right shifts limit the shift operand. */
> vect_convert_inputs (vinfo, last_stmt_info, nops, &ops[first_op - 1],
> op_type, &unprom[0], op_vectype);
>
> + /* Limit shift operands. */
> + if (code == RSHIFT_EXPR)
> + {
> + wide_int min_value, max_value;
> + if (TREE_CODE (ops[1]) == INTEGER_CST)
> + ops[1] = wide_int_to_tree (op_type,
> + wi::bit_and (wi::to_wide (ops[1]),
> + new_precision - 1));
> + else if (!vect_get_range_info (ops[1], &min_value, &max_value)
> + || wi::ge_p (max_value, new_precision, TYPE_SIGN (op_type)))
> + {
> + /* ??? Note the following bad for SLP as that only supports
> + same argument widened shifts and it un-CSEs same arguments. */
> + tree new_var = vect_recog_temp_ssa_var (op_type, NULL);
> + gimple *pattern_stmt
> + = gimple_build_assign (new_var, BIT_AND_EXPR, ops[1],
> + build_int_cst (op_type, new_precision - 1));
> + ops[1] = new_var;
> + gimple_set_location (pattern_stmt, gimple_location (last_stmt));
> + append_pattern_def_seq (vinfo, last_stmt_info, pattern_stmt);
> + }
> + }
> +
> /* Use the operation to produce a result of type OP_TYPE. */
> tree new_var = vect_recog_temp_ssa_var (op_type, NULL);
> gimple *pattern_stmt = gimple_build_assign (new_var, code,
