On Thu, 10 Sep 2026 at 6:32 AM, Andrea Pinski < [email protected]> wrote:
> On Sun, Sep 6, 2026 at 6:07 PM Shivam Gupta <[email protected]> > wrote: > > > > is_rshift_by_1 previously checked TYPE_UNSIGNED on the shift statement, > > preventing signed arithmetic right shifts from being optimized into CLZ > > calls. However, an arithmetic right shift behaves identically to a > logical > > right shift when the initial operand entering the loop is non-negative. > > > > This patch removes the strict TYPE_UNSIGNED requirement from > is_rshift_by_1 > > and adds ranger range_on_edge non-negativity checks in > number_of_iterations_cltz > > and number_of_iterations_cltz_complement on the loop preheader edge. > > > Ok. I forgot if you have write after approval access or not. > > Thank you, I do not have write after approval access but it would be nice to have someday. I plan to contribute to GCC in long term. > > > > PR tree-optimization/125734 > > > > gcc/ChangeLog: > > > > * tree-ssa-loop-niter.cc (is_rshift_by_1): Remove TYPE_UNSIGNED > restriction. > > (number_of_iterations_cltz): Use range_on_edge to check > non-negativity of > > signed preheader input. > > (number_of_iterations_cltz_complement): Likewise. > > > > gcc/testsuite/ChangeLog: > > > > * gcc.dg/tree-ssa/pr125734.c: New test. > > > > Regression tested on arch64-unknown-linux-gnu. > > > > Signed-off-by: Shivam Gupta <[email protected]> > > --- > > gcc/testsuite/gcc.dg/tree-ssa/pr125734.c | 32 ++++++++++++++++++++++++ > > gcc/tree-ssa-loop-niter.cc | 26 +++++++++++++++++-- > > 2 files changed, 56 insertions(+), 2 deletions(-) > > create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/pr125734.c > > > > diff --git a/gcc/testsuite/gcc.dg/tree-ssa/pr125734.c > b/gcc/testsuite/gcc.dg/tree-ssa/pr125734.c > > new file mode 100644 > > index 000000000..f8d76c459 > > --- /dev/null > > +++ b/gcc/testsuite/gcc.dg/tree-ssa/pr125734.c > > @@ -0,0 +1,32 @@ > > +/* { dg-do compile } */ > > +/* { dg-options "-O2 -fdump-tree-optimized" } */ > > + > > +int > > +nlz32 (int b) > > +{ > > + int c = sizeof(b) * 8; > > + if (b < 0) > > + return -1; > > + while (b != 0) > > + { > > + b >>= 1; > > + c--; > > + } > > + return c; > > +} > > + > > +int > > +nlz32_negative (int b) > > +{ > > + int c = sizeof(b) * 8; > > + > > + while (b != 0) > > + { > > + b >>= 1; > > + c--; > > + } > > + > > + return c; > > +} > > + > > +/* { dg-final { scan-tree-dump-times "\\.CLZ" 1 "optimized" } } */ > > diff --git a/gcc/tree-ssa-loop-niter.cc b/gcc/tree-ssa-loop-niter.cc > > index 0c1ebd1c7..3ce3dccfa 100644 > > --- a/gcc/tree-ssa-loop-niter.cc > > +++ b/gcc/tree-ssa-loop-niter.cc > > @@ -2313,8 +2313,6 @@ is_lshift_by_1 (gassign *stmt) > > static bool > > is_rshift_by_1 (gassign *stmt) > > { > > - if (!TYPE_UNSIGNED (TREE_TYPE (gimple_assign_lhs (stmt)))) > > - return false; > > if (gimple_assign_rhs_code (stmt) == RSHIFT_EXPR > > && integer_onep (gimple_assign_rhs2 (stmt))) > > return true; > > @@ -2482,6 +2480,18 @@ number_of_iterations_cltz (loop_p loop, edge exit, > > > > /* We found a match. */ > > tree src = gimple_phi_arg_def (phi, loop_preheader_edge > (loop)->dest_idx); > > + > > + /* If the type is signed, verify via Ranger on the preheader edge > > + that the initial value entering the loop is non-negative. */ > > + if (!TYPE_UNSIGNED (TREE_TYPE (src))) > > + { > > + int_range_max r (TREE_TYPE (src)); > > + edge e = loop_preheader_edge (loop); > > + if (!get_range_query (cfun)->range_on_edge (r, e, src) > > + || !r.nonnegative_p ()) > > + return false; > > + } > > + > > int src_precision = TYPE_PRECISION (TREE_TYPE (src)); > > > > /* Save the original SSA name before preprocessing for ranger > queries. */ > > @@ -2641,6 +2651,18 @@ number_of_iterations_cltz_complement (loop_p > loop, edge exit, > > > > /* We found a match. */ > > tree src = gimple_phi_arg_def (phi, loop_preheader_edge > (loop)->dest_idx); > > + > > + /* If the type is signed, verify via Ranger on the preheader edge > > + that the initial value entering the loop is non-negative. */ > > + if (!TYPE_UNSIGNED (TREE_TYPE (src))) > > + { > > + int_range_max r (TREE_TYPE (src)); > > + edge e = loop_preheader_edge (loop); > > + if (!get_range_query (cfun)->range_on_edge (r, e, src) > > + || !r.nonnegative_p ()) > > + return false; > > + } > > + > > int src_precision = TYPE_PRECISION (TREE_TYPE (src)); > > > > /* Get the corresponding c[lt]z builtin. */ > > -- > > 2.43.0 > > >
