As observed by Jakub in comment #2 of PR 98865, the expression -(a>>63)
is optimized in GENERIC but not in GIMPLE. Investigating further it
turns out that this is one of a few transformations performed by
fold_negate_expr in fold-const.c that aren't yet performed by match.pd.
This patch moves/duplicates them there, and should be relatively safe
as these transformations are already performed by the compiler, but
just in different passes.
Alas the one minor complication is that some of these transformations
are only wins, if the intermediate result (of the multiplication or
division) is only used once, to avoid duplication/performing them again.
See gcc.dg/tree-ssa/ssa-free-88.c. Normally, this is the perfect usage
of match's single_use (aka SSA's has_single_use). Alas, single_use is
not always accurate in match.pd, as some passes will construct and
simplify an expression/stmt before inserting it into GIMPLE, and folding
during this process sees the temporary undercount from the data-flow.
To solve this, this patch introduces a new single_use_is_op_p that
double checks that the single_use has the expected tree_code/operation
and skips the transformation if we can tell single_use might be invalid.
A follow-up patch might be to investigate whether genmatch.c can be
tweaked to use this new helper function to implement the :s qualifier
when the enclosing context is/should be known, but that's overkill
to just unblock Jakub and Andrew on 98865.
This patch has been tested on x86_64-pc-linux-gnu with a "make bootstrap"
and "make -k check" with no new failures. Ok for mainline?
2021-09-09 Roger Sayle <ro...@nextmovesoftware.com>
gcc/ChangeLog
* generic-match-head.c (single_use_is_op_p): New helper function.
* gimple-match-head.c (single_use_is_op_p): New helper function.
* match.pd (negation simplifications): Implement some negation
folding transformations from fold-const.c's fold_negate_expr.
gcc/testsuite/ChangeLog
* gcc.dg/fold-negate-1.c: New test case.
Roger
--
diff --git a/gcc/generic-match-head.c b/gcc/generic-match-head.c
index f426208..64115a2 100644
--- a/gcc/generic-match-head.c
+++ b/gcc/generic-match-head.c
@@ -63,6 +63,16 @@ single_use (tree t ATTRIBUTE_UNUSED)
return true;
}
+/* Like single_use above, but confirm that the single use (if any)
+ is an expression of tree_code OP. For GENERIC, assume the worst
+ and postpone folding transformations until GIMPLE. */
+
+static inline bool
+single_use_is_op_p (tree t ATTRIBUTE_UNUSED, tree_code op ATTRIBUTE_UNUSED)
+{
+ return false;
+}
+
/* Return true if math operations should be canonicalized,
e.g. sqrt(sqrt(x)) -> pow(x, 0.25). */
diff --git a/gcc/gimple-match-head.c b/gcc/gimple-match-head.c
index 7112c11..2026f96a 100644
--- a/gcc/gimple-match-head.c
+++ b/gcc/gimple-match-head.c
@@ -1141,6 +1141,22 @@ single_use (tree t)
return TREE_CODE (t) != SSA_NAME || has_zero_uses (t) || has_single_use (t);
}
+/* Like single_use above, but confirm that the single use (if any)
+ is an expression of tree_code OP. */
+
+static inline bool
+single_use_is_op_p (tree t, tree_code op)
+{
+ use_operand_p use;
+ gimple *stmt;
+
+ return TREE_CODE (t) != SSA_NAME
+ || has_zero_uses (t)
+ || (single_imm_use (t, &use, &stmt)
+ && is_gimple_assign (stmt)
+ && gimple_assign_rhs_code (stmt) == op);
+}
+
/* Return true if math operations should be canonicalized,
e.g. sqrt(sqrt(x)) -> pow(x, 0.25). */
diff --git a/gcc/match.pd b/gcc/match.pd
index 008f775..091da6c 100644
--- a/gcc/match.pd
+++ b/gcc/match.pd
@@ -1481,6 +1481,36 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
(if (!FIXED_POINT_TYPE_P (type))
(plus @0 (negate @1))))
+/* Other simplifications of negation (c.f. fold_negate_expr_1). */
+(simplify
+ (negate (mult:c@0 @1 negate_expr_p@2))
+ (if (! TYPE_UNSIGNED (type)
+ && ! HONOR_SIGN_DEPENDENT_ROUNDING (type)
+ && single_use_is_op_p (@0, NEGATE_EXPR))
+ (mult @1 (negate @2))))
+
+(simplify
+ (negate (rdiv@0 @1 negate_expr_p@2))
+ (if (! HONOR_SIGN_DEPENDENT_ROUNDING (type)
+ && single_use_is_op_p (@0, NEGATE_EXPR))
+ (rdiv @1 (negate @2))))
+
+(simplify
+ (negate (rdiv@0 negate_expr_p@1 @2))
+ (if (! HONOR_SIGN_DEPENDENT_ROUNDING (type)
+ && single_use_is_op_p (@0, NEGATE_EXPR))
+ (rdiv (negate @1) @2)))
+
+/* Fold -((int)x >> (prec - 1)) into (unsigned)x >> (prec - 1). */
+(simplify
+ (negate (convert? (rshift @0 INTEGER_CST@1)))
+ (if (tree_nop_conversion_p (type, TREE_TYPE (@0))
+ && wi::to_wide (@1) == element_precision (type) - 1)
+ (with { tree stype = TREE_TYPE (@0);
+ tree ntype = TYPE_UNSIGNED (stype) ? signed_type_for (stype)
+ : unsigned_type_for (stype); }
+ (convert (rshift:ntype (convert:ntype @0) @1)))))
+
/* Try to fold (type) X op CST -> (type) (X op ((type-x) CST))
when profitable.
For bitwise binary operations apply operand conversions to the
/* { dg-do compile } */
/* { dg-options "-O2 -fdump-tree-optimized" } */
#define SHIFT ((8*__SIZEOF_INT__)-1)
int test_rshift_1(int x)
{
int t = x >> SHIFT;
return -t;
}
int test_rshift_2(int x)
{
unsigned int t = (unsigned int)x >> SHIFT;
return -t;
}
int test_rshift_3(int x)
{
int t = (unsigned int)x >> SHIFT;
return -t;
}
int test_rshift_4(int x)
{
unsigned int t = x >> SHIFT;
return -t;
}
double test_mul_1(double x)
{
double t = -5.0 * x;
return -t;
}
double test_mul_2(double x, double y)
{
double t1 = -x;
double t2 = t1 * y;
return -t2;
}
double test_rdiv_1(double x, double y)
{
double t1 = -x;
double t2 = t1 / y;
return -t2;
}
double test_rdiv_2(double x, double y)
{
double t1 = -y;
double t2 = x / t1;
return -t2;
}
/* { dg-final { scan-tree-dump-not " -" "optimized" } } */
