| Message ID | 1497273586-9046-1-git-send-email-james.greenhalgh@arm.com |
|---|---|
| State | New |
| Headers | show |
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. 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). Richard. > Thanks, > James > > --- > gcc/ > > 2017-06-12 James Greenhalgh <james.greenhalgh@arm.com> > > * match.pd (A / (1 << B) -> A >> B): New. > > gcc/testsuite/ > > 2017-06-12 James Greenhalgh <james.greenhalgh@arm.com> > > * gcc.dg/tree-ssa/forwprop-37.c: New. > >
diff --git a/gcc/match.pd b/gcc/match.pd index 54a8e04..656ede2 100644 --- a/gcc/match.pd +++ b/gcc/match.pd @@ -147,6 +147,14 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT) (op @0 integer_onep) (non_lvalue @0))) +/* For unsigned A: (A / (1 << B)), (A >> B). + We can't do the same for signed A, as it might be negative, which would + introduce undefined behaviour. */ +(simplify + (trunc_div @0 (lshift integer_onep@1 @2)) + (if (TYPE_UNSIGNED (type)) + (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" } } */
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? Thanks, James --- gcc/ 2017-06-12 James Greenhalgh <james.greenhalgh@arm.com> * match.pd (A / (1 << B) -> A >> B): New. gcc/testsuite/ 2017-06-12 James Greenhalgh <james.greenhalgh@arm.com> * gcc.dg/tree-ssa/forwprop-37.c: New.