On 05/10/2021 13:50, Tamar Christina via Gcc-patches wrote:
Hi All,
This turns an inversion of the sign bit + arithmetic right shift into a
comparison with 0.
i.e.
void fun1(int32_t *x, int n)
{
for (int i = 0; i < (n & -16); i++)
x[i] = (-x[i]) >> 31;
}
Notwithstanding that I think shifting a negative value right is
unspecified behaviour, I don't think this generates the same result when
x[i] is INT_MIN either, although negating that is also unspecified since
it can't be represented in an int.
R.
now generates:
.L3:
ldr q0, [x0]
cmgt v0.4s, v0.4s, #0
str q0, [x0], 16
cmp x0, x1
bne .L3
instead of:
.L3:
ldr q0, [x0]
neg v0.4s, v0.4s
sshr v0.4s, v0.4s, 31
str q0, [x0], 16
cmp x0, x1
bne .L3
Bootstrapped Regtested on aarch64-none-linux-gnu,
x86_64-pc-linux-gnu and no regressions.
Ok for master?
Thanks,
Tamar
gcc/ChangeLog:
* match.pd: New negate+shift pattern.
gcc/testsuite/ChangeLog:
* gcc.dg/signbit-2.c: New test.
* gcc.dg/signbit-3.c: New test.
* gcc.target/aarch64/signbit-1.c: New test.
--- inline copy of patch --
diff --git a/gcc/match.pd b/gcc/match.pd
index
7d2a24dbc5e9644a09968f877e12a824d8ba1caa..581436fe36dbacdcb0c2720b7190c96d14398143
100644
--- a/gcc/match.pd
+++ b/gcc/match.pd
@@ -826,6 +826,37 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
{ tree utype = unsigned_type_for (type); }
(convert (rshift (lshift (convert:utype @0) @2) @3))))))
+/* Fold (-x >> C) into x > 0 where C = precision(type) - 1. */
+(for cst (INTEGER_CST VECTOR_CST)
+ (simplify
+ (rshift (negate:s @0) cst@1)
+ (with { tree ctype = TREE_TYPE (@0);
+ tree stype = TREE_TYPE (@1);
+ tree bt = truth_type_for (ctype); }
+ (switch
+ /* Handle scalar case. */
+ (if (INTEGRAL_TYPE_P (ctype)
+ && !VECTOR_TYPE_P (ctype)
+ && !TYPE_UNSIGNED (ctype)
+ && canonicalize_math_after_vectorization_p ()
+ && wi::eq_p (wi::to_wide (@1), TYPE_PRECISION (stype) - 1))
+ (convert:bt (gt:bt @0 { build_zero_cst (stype); })))
+ /* Handle vector case with a scalar immediate. */
+ (if (VECTOR_INTEGER_TYPE_P (ctype)
+ && !VECTOR_TYPE_P (stype)
+ && !TYPE_UNSIGNED (ctype)
+ && wi::eq_p (wi::to_wide (@1), TYPE_PRECISION (stype) - 1))
+ (convert:bt (gt:bt @0 { build_zero_cst (ctype); })))
+ /* Handle vector case with a vector immediate. */
+ (if (VECTOR_INTEGER_TYPE_P (ctype)
+ && VECTOR_TYPE_P (stype)
+ && !TYPE_UNSIGNED (ctype)
+ && uniform_vector_p (@1))
+ (with { tree cst = vector_cst_elt (@1, 0);
+ tree t = TREE_TYPE (cst); }
+ (if (wi::eq_p (wi::to_wide (cst), TYPE_PRECISION (t) - 1))
+ (convert:bt (gt:bt @0 { build_zero_cst (ctype); })))))))))
+
/* Fold (C1/X)*C2 into (C1*C2)/X. */
(simplify
(mult (rdiv@3 REAL_CST@0 @1) REAL_CST@2)
diff --git a/gcc/testsuite/gcc.dg/signbit-2.c b/gcc/testsuite/gcc.dg/signbit-2.c
new file mode 100644
index
0000000000000000000000000000000000000000..fc0157cbc5c7996b481f2998bc30176c96a669bb
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/signbit-2.c
@@ -0,0 +1,19 @@
+/* { dg-do assemble } */
+/* { dg-options "-O3 --save-temps -fdump-tree-optimized" } */
+
+#include <stdint.h>
+
+void fun1(int32_t *x, int n)
+{
+ for (int i = 0; i < (n & -16); i++)
+ x[i] = (-x[i]) >> 31;
+}
+
+void fun2(int32_t *x, int n)
+{
+ for (int i = 0; i < (n & -16); i++)
+ x[i] = (-x[i]) >> 30;
+}
+
+/* { dg-final { scan-tree-dump-times {\s+>\s+\{ 0, 0, 0, 0 \}} 1 optimized } }
*/
+/* { dg-final { scan-tree-dump-not {\s+>>\s+31} optimized } } */
diff --git a/gcc/testsuite/gcc.dg/signbit-3.c b/gcc/testsuite/gcc.dg/signbit-3.c
new file mode 100644
index
0000000000000000000000000000000000000000..19e9c06c349b3287610f817628f00938ece60bf7
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/signbit-3.c
@@ -0,0 +1,13 @@
+/* { dg-do assemble } */
+/* { dg-options "-O1 --save-temps -fdump-tree-optimized" } */
+
+#include <stdint.h>
+
+void fun1(int32_t *x, int n)
+{
+ for (int i = 0; i < (n & -16); i++)
+ x[i] = (-x[i]) >> 31;
+}
+
+/* { dg-final { scan-tree-dump-times {\s+>\s+0;} 1 optimized } } */
+/* { dg-final { scan-tree-dump-not {\s+>>\s+31} optimized } } */
diff --git a/gcc/testsuite/gcc.target/aarch64/signbit-1.c
b/gcc/testsuite/gcc.target/aarch64/signbit-1.c
new file mode 100644
index
0000000000000000000000000000000000000000..3ebfb0586f37de29cf58635b27fe48503714447e
--- /dev/null
+++ b/gcc/testsuite/gcc.target/aarch64/signbit-1.c
@@ -0,0 +1,18 @@
+/* { dg-do assemble } */
+/* { dg-options "-O3 --save-temps" } */
+
+#include <stdint.h>
+
+void fun1(int32_t *x, int n)
+{
+ for (int i = 0; i < (n & -16); i++)
+ x[i] = (-x[i]) >> 31;
+}
+
+void fun2(int32_t *x, int n)
+{
+ for (int i = 0; i < (n & -16); i++)
+ x[i] = (-x[i]) >> 30;
+}
+
+/* { dg-final { scan-assembler-times {\tcmgt\t} 1 } } */
