| Message ID | 20210831073243.GR920497@tucnak |
|---|---|
| State | New |
| Headers | show |
| Series | tree-ssa-ccp: Fix up bit_value_binop on RSHIFT_EXPR [PR102134] | expand |
On Tue, 31 Aug 2021, Jakub Jelinek wrote: > Hi! > > 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. > > Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk? OK. > 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. > > --- gcc/tree-ssa-ccp.c.jj 2021-08-30 08:36:11.302515439 +0200 > +++ gcc/tree-ssa-ccp.c 2021-08-30 22:49:21.957503630 +0200 > @@ -1695,7 +1695,8 @@ bit_value_binop (enum tree_code code, si > /* 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)); > --- gcc/testsuite/gcc.c-torture/execute/pr102134.c.jj 2021-08-30 22:47:41.115920522 +0200 > +++ gcc/testsuite/gcc.c-torture/execute/pr102134.c 2021-08-30 22:47:20.811205820 +0200 > @@ -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; > +} > > Jakub > >
--- gcc/tree-ssa-ccp.c.jj 2021-08-30 08:36:11.302515439 +0200 +++ gcc/tree-ssa-ccp.c 2021-08-30 22:49:21.957503630 +0200 @@ -1695,7 +1695,8 @@ bit_value_binop (enum tree_code code, si /* 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)); --- gcc/testsuite/gcc.c-torture/execute/pr102134.c.jj 2021-08-30 22:47:41.115920522 +0200 +++ gcc/testsuite/gcc.c-torture/execute/pr102134.c 2021-08-30 22:47:20.811205820 +0200 @@ -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; +}