On Fri, Sep 23, 2022 at 2:26 AM Tamar Christina via Gcc-patches <gcc-patches@gcc.gnu.org> wrote: > > Hi All, > > This is an RFC to figure out how to deal with targets that don't have native > comparisons against QImode values. > > Booleans, at least in C99 and higher are 0-1 valued. This means that we only > really need to test a single bit. However in RTL we no longer have this > information available and just have an SImode value (due to the promotion of > QImode to SImode). > > This RFC fixes it by emitting an explicit & 1 during the expansion of the > conditional branch. > > However it's unlikely that we want to do this unconditionally. Most targets > I've tested seem to have harmless code changes, like x86 changes from testb to > andl, $1. > > So I have two questions: > > 1. Should I limit this behind a target macro? Or should I just leave it for > all > targets and deal with the fallout. > 2. How can I tell whether the C99 0-1 values bools are being used or the older > 0, non-0 variant? > > Bootstrapped Regtested on aarch64-none-linux-gnu and no issues. > However there are some benign codegen changes on x86, testb changed to andl > $1. > > This pattern occurs more than 120,000 times in SPECCPU 2017 and so is quite > common. > > Thanks, > Tamar > > gcc/ChangeLog: > > * tree.h (tree_zero_one_valued_p): New. > * dojump.cc (do_jump): Add & 1 if truth type.
Just FYI, I think this fixes/improves https://gcc.gnu.org/PR101806 . Thanks, Andrew > > --- inline copy of patch -- > diff --git a/gcc/dojump.cc b/gcc/dojump.cc > index > 2af0cd1aca3b6af13d5d8799094ee93f18022296..8eaf1be49cd12298e61c6946ae79ca9de6197864 > 100644 > --- a/gcc/dojump.cc > +++ b/gcc/dojump.cc > @@ -605,7 +605,17 @@ do_jump (tree exp, rtx_code_label *if_false_label, > /* Fall through and generate the normal code. */ > default: > normal: > - temp = expand_normal (exp); > + tree cmp = exp; > + /* If the expression is a truth type then explicitly generate an & 1 > + to indicate to the target that it's a zero-one values type. This > + allows the target to further optimize the comparison should it > + choose to. */ > + if (tree_zero_one_valued_p (exp)) > + { > + type = TREE_TYPE (exp); > + cmp = build2 (BIT_AND_EXPR, type, exp, build_int_cstu (type, 1)); > + } > + temp = expand_normal (cmp); > do_pending_stack_adjust (); > /* The RTL optimizers prefer comparisons against pseudos. */ > if (GET_CODE (temp) == SUBREG) > diff --git a/gcc/tree.h b/gcc/tree.h > index > 8f8a9660c9e0605eb516de194640b8c1b531b798..be3d2dee82f692e81082cf21c878c10f9fe9e1f1 > 100644 > --- a/gcc/tree.h > +++ b/gcc/tree.h > @@ -4690,6 +4690,7 @@ extern tree signed_or_unsigned_type_for (int, tree); > extern tree signed_type_for (tree); > extern tree unsigned_type_for (tree); > extern bool is_truth_type_for (tree, tree); > +extern bool tree_zero_one_valued_p (tree); > extern tree truth_type_for (tree); > extern tree build_pointer_type_for_mode (tree, machine_mode, bool); > extern tree build_pointer_type (tree); > > > > > --
