OpenE2K/gcc
2
0
Fork 0
Code Issues Pull Requests Projects Releases Wiki Activity

tree-ssa-ccp: Fix up bit_value_binop on RSHIFT_EXPR [PR102134]

Browse Source 
As mentioned in the PR, this hunk is guarded with !wi::neg_p (r1val | r1mask, sgn)
which means if sgn is UNSIGNED, it is always true, but r1val | r1mask in
widest_int is still sign-extended.  That means wi::clz (arg) returns 0,
wi::get_precision (arg) returns some very large number
(WIDE_INT_MAX_PRECISION, on x86_64 576 bits) and width is 64, so we end up
with lzcount of -512 where the code afterwards expects a non-negative
lzcount.  For arg without the sign bit set the code works right, those
numbers are zero extended and so wi::clz must return wi::get_precision (arg) - width
plus number of leading zero bits within the width precision.
The patch fixes it by handling the sign-extension specially, either it could
be done through wi::neg_p (arg) check, but lzcount == 0 works identically.

2021-08-31  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/102134
	* tree-ssa-ccp.c (bit_value_binop) <case RSHIFT_EXPR>: If sgn is
	UNSIGNED and r1val | r1mask has MSB set, ensure lzcount doesn't
	become negative.

	* gcc.c-torture/execute/pr102134.c: New test.
This commit is contained in:
Jakub Jelinek 2021-08-31 10:29:23 +02:00
parent dd817ccd26
commit 06ac981ffb
2 changed files with 25 additions and 1 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

23
gcc/testsuite/gcc.c-torture/execute/pr102134.c Normal file
View File

@ -0,0 +1,23 @@
/* PR tree-optimization/102134 */
typedef unsigned long long u64;
u64 g;
void
foo (u64 a, u64 b, u64 c, u64 *r)
{
b *= b;
u64 x = a && ((b >> (c & 63)) | ((b << (c & 63)) & g));
*r = x + a;
}
int
main ()
{
u64 x;
foo (1, 3000, 0, &x);
if (x != 2)
__builtin_abort ();
return 0;
}

3
gcc/tree-ssa-ccp.c
View File

@ -1695,7 +1695,8 @@ bit_value_binop (enum tree_code code, signop sgn, int width,
/* Logical right shift, or zero sign bit. */
widest_int arg = r1val | r1mask;
int lzcount = wi::clz (arg);
lzcount -= wi::get_precision (arg) - width;
if (lzcount)
lzcount -= wi::get_precision (arg) - width;
widest_int tmp = wi::mask <widest_int> (width, false);
tmp = wi::lrshift (tmp, lzcount);
tmp = wi::lrshift (tmp, wi::bit_and_not (r2val, r2mask));