[PATCH] middle-end: fix de-optimizations with bitclear patterns on signed values
Richard Biener
rguenther@suse.de
Tue Oct 26 08:26:18 GMT 2021
On Mon, 25 Oct 2021, Tamar Christina wrote:
> > -----Original Message-----
> > From: Richard Biener <rguenther@suse.de>
> > Sent: Friday, October 15, 2021 12:31 PM
> > To: Tamar Christina <Tamar.Christina@arm.com>
> > Cc: gcc-patches@gcc.gnu.org; Jakub Jelinek <jakub@redhat.com>; nd
> > <nd@arm.com>
> > Subject: Re: [PATCH] middle-end: fix de-optimizations with bitclear patterns
> > on signed values
> >
> > On Fri, 15 Oct 2021, Tamar Christina wrote:
> >
> > > Hi All,
> > >
> > > During testing after rebasing to commit I noticed a failing testcase
> > > with the bitmask compare patch.
> > >
> > > Consider the following C++ testcase:
> > >
> > > #include <compare>
> > >
> > > #define A __attribute__((noipa))
> > > A bool f5 (double i, double j) { auto c = i <=> j; return c >= 0; }
> > >
> > > This turns into a comparison against chars, on systems where chars are
> > > signed the pattern inserts an unsigned convert such that it's able to
> > > do the transformation.
> > >
> > > i.e.:
> > >
> > > # RANGE [-1, 2]
> > > # c$_M_value_22 = PHI <-1(3), 0(2), 2(5), 1(4)>
> > > # RANGE ~[3, 254]
> > > _11 = (unsigned char) c$_M_value_22;
> > > _19 = _11 <= 1;
> > > # .MEM_24 = VDEF <.MEM_6(D)>
> > > D.10434 ={v} {CLOBBER};
> > > # .MEM_14 = VDEF <.MEM_24>
> > > D.10407 ={v} {CLOBBER};
> > > # VUSE <.MEM_14>
> > > return _19;
> > >
> > > instead of:
> > >
> > > # RANGE [-1, 2]
> > > # c$_M_value_5 = PHI <-1(3), 0(2), 2(5), 1(4)>
> > > # RANGE [-2, 2]
> > > _3 = c$_M_value_5 & -2;
> > > _19 = _3 == 0;
> > > # .MEM_24 = VDEF <.MEM_6(D)>
> > > D.10440 ={v} {CLOBBER};
> > > # .MEM_14 = VDEF <.MEM_24>
> > > D.10413 ={v} {CLOBBER};
> > > # VUSE <.MEM_14>
> > > return _19;
> > >
> > > This causes much worse codegen under -ffast-math due to phiops no
> > > longer recognizing the pattern. It turns out that phiopts
> > > spaceship_replacement is looking for the exact form that was just changed.
> > >
> > > Trying to get it to recognize the new form is not trivial as the
> > > transformation doesn't look to work when the thing it's pointing to is itself
> > a phi-node.
> >
> > What do you mean? Where it handles the BIT_AND it could also handle the
> > conversion, no? The later handling would probably more explicitely need to
> > distinguish between the BIT_AND and the conversion forms.
>
> Looks like I misunderstood the code, it was looking at the uses not the defs of
> the value.
>
> --- inline copy of patch ---
>
> The comments seems to suggest this code only checks for (res & ~1) == 0 but the
> implementation seems to suggest it's broader.
>
> As such I added a case to check to see if the value comparison we found is a
> type cast. and strips away the type cast and continues.
>
> In match.pd the typecasts are only added for signed comparisons to == 0 and != 0
> which are then rewritten into comparisons with 1.
>
> As such I only check for 1 and LE and GT, which is what match.pd would have
> rewritten it to.
>
> This fixes the regression but this is not code I 100% understand, since I don't
> really know the semantics of the spaceship operator so would appreciate an extra
> look.
>
> Bootstrapped Regtested on aarch64-none-linux-gnu,
> x86_64-pc-linux-gnu and no regressions.
>
> Ok for master?
Please add a testcase. I hope Jakub can review the spaceship_replacement
patch since he's the one familiar with the code.
Thanks,
Richard.
> Thanks,
> Tamar
>
> gcc/ChangeLog:
>
> * tree-ssa-phiopt.c (spaceship_replacement): Handle new canonical
> codegen.
>
> diff --git a/gcc/tree-ssa-phiopt.c b/gcc/tree-ssa-phiopt.c
> index 0e339c46afa29fa97f90d9bc4394370cd9b4b396..65b25be3399b75d5e9cab0f78aa2340418571a33 100644
> --- a/gcc/tree-ssa-phiopt.c
> +++ b/gcc/tree-ssa-phiopt.c
> @@ -2037,6 +2037,7 @@ spaceship_replacement (basic_block cond_bb, basic_block middle_bb,
> tree lhs, rhs;
> gimple *orig_use_stmt = use_stmt;
> tree orig_use_lhs = NULL_TREE;
> + bool is_canon = false;
> int prec = TYPE_PRECISION (TREE_TYPE (phires));
> if (is_gimple_assign (use_stmt)
> && gimple_assign_rhs_code (use_stmt) == BIT_AND_EXPR
> @@ -2063,6 +2064,26 @@ spaceship_replacement (basic_block cond_bb, basic_block middle_bb,
> }
> else if (is_gimple_assign (use_stmt))
> {
> + /* Deal with if match.pd has rewritten the (res & ~1) == 0
> + into res <= 1 and has left a type-cast for signed types. */
> + if (gimple_assign_cast_p (use_stmt))
> + {
> + orig_use_lhs = gimple_assign_lhs (use_stmt);
> + if (SSA_NAME_OCCURS_IN_ABNORMAL_PHI (orig_use_lhs))
> + return false;
> + if (EDGE_COUNT (phi_bb->preds) != 4)
> + return false;
> + if (!TYPE_UNSIGNED (TREE_TYPE (orig_use_lhs)))
> + return false;
> + if (!single_imm_use (orig_use_lhs, &use_p, &use_stmt))
> + return false;
> + tree_code cmp;
> + if (is_gimple_assign (use_stmt)
> + && (cmp = gimple_assign_rhs_code (use_stmt))
> + && (cmp == LE_EXPR || cmp == GT_EXPR)
> + && wi::eq_p (wi::to_wide (gimple_assign_rhs2 (use_stmt)), 1))
> + is_canon = true;
> + }
> if (gimple_assign_rhs_class (use_stmt) == GIMPLE_BINARY_RHS)
> {
> cmp = gimple_assign_rhs_code (use_stmt);
> @@ -2099,7 +2120,9 @@ spaceship_replacement (basic_block cond_bb, basic_block middle_bb,
> || !tree_fits_shwi_p (rhs)
> || !IN_RANGE (tree_to_shwi (rhs), -1, 1))
> return false;
> - if (orig_use_lhs)
> + /* If we're already in the canonical form we need to keep the original
> + comparison. */
> + if (orig_use_lhs && !is_canon)
> {
> if ((cmp != EQ_EXPR && cmp != NE_EXPR) || !integer_zerop (rhs))
> return false;
> @@ -2310,6 +2333,7 @@ spaceship_replacement (basic_block cond_bb, basic_block middle_bb,
> one_cmp = GT_EXPR;
>
> enum tree_code res_cmp;
> +
> switch (cmp)
> {
> case EQ_EXPR:
> @@ -2345,6 +2369,8 @@ spaceship_replacement (basic_block cond_bb, basic_block middle_bb,
> res_cmp = one_cmp == LT_EXPR ? GE_EXPR : LE_EXPR;
> else if (integer_minus_onep (rhs))
> res_cmp = one_cmp == LT_EXPR ? GT_EXPR : LT_EXPR;
> + else if (integer_onep (rhs) && is_canon)
> + res_cmp = GE_EXPR;
> else
> return false;
> break;
> @@ -2353,6 +2379,8 @@ spaceship_replacement (basic_block cond_bb, basic_block middle_bb,
> res_cmp = one_cmp == LT_EXPR ? LE_EXPR : GE_EXPR;
> else if (integer_zerop (rhs))
> res_cmp = one_cmp;
> + else if (integer_onep (rhs) && is_canon)
> + res_cmp = LE_EXPR;
> else
> return false;
> break;
>
--
Richard Biener <rguenther@suse.de>
SUSE Software Solutions Germany GmbH, Maxfeldstrasse 5, 90409 Nuernberg,
Germany; GF: Felix Imendörffer; HRB 36809 (AG Nuernberg)
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