| Message ID | 1497604990-5082-1-git-send-email-james.greenhalgh@arm.com |
|---|---|
| State | New |
| Headers | show |
On Fri, 16 Jun 2017, James Greenhalgh wrote: > > On Mon, Jun 12, 2017 at 03:56:25PM +0200, Richard Biener wrote: > > On Mon, 12 Jun 2017, James Greenhalgh wrote: > > > > > > > > Hi, > > > > > > As subject, for the testcase in the patch: > > > > > > unsigned long > > > f2 (unsigned long a, int b) > > > { > > > unsigned long x = 1UL << b; > > > return a / x; > > > } > > > > > > We currently generate: > > > > > > f2: > > > mov x2, 1 > > > lsl x1, x2, x1 > > > udiv x0, x0, x1 > > > ret > > > > > > Which could instead be transformed to: > > > > > > f2: > > > lsr x0, x0, x1 > > > ret > > > > > > OK? > > > > + We can't do the same for signed A, as it might be negative, which > > would > > + introduce undefined behaviour. */ > > > > huh, AFAIR it is _left_ shift of negative values that invokes > > undefined behavior. > > You're right this is not a clear comment. The problem is not undefined > behaviour, so that text needs to go, but rounding towards/away from zero > for signed negative values. Division will round towards zero, arithmetic > right shift away from zero. For example in: > > -1 / (1 << 1) != -1 >> 1 > = -1 / 2 > = 0 = -1 > > I've rewritten the comment to make it clear this is why we can only make > this optimisation for unsigned values. Ah, of course. You could use if ((TYPE_UNSIGNED (type) || tree_expr_nonnegative_p (@0)) here as improvement. > See, for example, gcc.c-torture/execute/pr34070-2.c > > > Note that as you are accepting vectors you need to make sure the > > target actually supports arithmetic right shift of vectors > > (you only know it supports left shift and division -- so it might > > be sort-of-superfluous to check in case there is no arch that supports > > those but not the other). > > I've added a check for that using optabs, is that the right way to do this? + && (!VECTOR_TYPE_P (type) + || optab_for_tree_code (RSHIFT_EXPR, type, optab_vector) + || optab_for_tree_code (RSHIFT_EXPR, type, optab_scalar))) is not enough -- you need sth like optab ot = optab_for_tree_code (RSHIFT_EXPR, type, optab_vector); if (ot != unknown_optab && optab_handler (ot, TYPE_MODE (type)) != CODE_FOR_nothing) .. ok! ... ideally we'd have a helper for this in optab-tree.[ch], tree-vect-patterns.c could also make use of that. Thanks, Richard. > Bootstrapped and tested on aarch64-none-linux-gnu with no issues. > > OK? > > Thanks, > James > > --- > gcc/ > > 2017-06-13 James Greenhalgh <james.greenhalgh@arm.com> > > * match.pd (A / (1 << B) -> A >> B): New. > * generic-match-head.c: Include optabs-tree.h. > * gimple-match-head.c: Likewise. > > gcc/testsuite/ > > 2017-06-13 James Greenhalgh <james.greenhalgh@arm.com> > > * gcc.dg/tree-ssa/forwprop-37.c: New. > >
diff --git a/gcc/generic-match-head.c b/gcc/generic-match-head.c index 0c0d182..4504401 100644 --- a/gcc/generic-match-head.c +++ b/gcc/generic-match-head.c @@ -33,6 +33,7 @@ along with GCC; see the file COPYING3. If not see #include "builtins.h" #include "case-cfn-macros.h" #include "gimplify.h" +#include "optabs-tree.h" /* Routine to determine if the types T1 and T2 are effectively diff --git a/gcc/gimple-match-head.c b/gcc/gimple-match-head.c index e7e9839..5f6aa27 100644 --- a/gcc/gimple-match-head.c +++ b/gcc/gimple-match-head.c @@ -39,6 +39,7 @@ along with GCC; see the file COPYING3. If not see #include "internal-fn.h" #include "case-cfn-macros.h" #include "gimplify.h" +#include "optabs-tree.h" /* Forward declarations of the private auto-generated matchers. diff --git a/gcc/match.pd b/gcc/match.pd index 244e9eb..2bea268 100644 --- a/gcc/match.pd +++ b/gcc/match.pd @@ -147,6 +147,18 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT) (op @0 integer_onep) (non_lvalue @0))) +/* (A / (1 << B)) -> (A >> B). + Only for unsigned A. For signed A, this would not preserve rounding + toward zero. + For example: (-1 / ( 1 << B)) != -1 >> B. */ +(simplify + (trunc_div @0 (lshift integer_onep@1 @2)) + (if (TYPE_UNSIGNED (type) + && (!VECTOR_TYPE_P (type) + || optab_for_tree_code (RSHIFT_EXPR, type, optab_vector) + || optab_for_tree_code (RSHIFT_EXPR, type, optab_scalar))) + (rshift @0 @2))) + /* Preserve explicit divisions by 0: the C++ front-end wants to detect undefined behavior in constexpr evaluation, and assuming that the division traps enables better optimizations than these anyway. */ diff --git a/gcc/testsuite/gcc.dg/tree-ssa/forwprop-37.c b/gcc/testsuite/gcc.dg/tree-ssa/forwprop-37.c new file mode 100644 index 0000000..dec826c --- /dev/null +++ b/gcc/testsuite/gcc.dg/tree-ssa/forwprop-37.c @@ -0,0 +1,25 @@ +/* { dg-do compile } */ +/* { dg-options "-O -fdump-tree-forwprop1-raw" } */ + +unsigned int +f1 (unsigned int a, unsigned int b) +{ + unsigned int x = 1U << b; + return a / x; +} + +unsigned long +f2 (unsigned long a, int b) +{ + unsigned long x = 1UL << b; + return a / x; +} + +unsigned long long +f3 (unsigned long long a, int b) +{ + unsigned long long x = 1ULL << b; + return a / x; +} + +/* { dg-final { scan-tree-dump-not "trunc_div_expr" "forwprop1" } } */