On 6/12/23 01:57, Stefan Schulze Frielinghaus via Gcc-patches wrote:
Comparisons between memory and constants might be done in a smaller mode
resulting in smaller constants which might finally end up as immediates
instead of in the literal pool.
For example, on s390x a non-symmetric comparison like
x <= 0x3fffffffffffffff
results in the constant being spilled to the literal pool and an 8 byte
memory comparison is emitted. Ideally, an equivalent comparison
x0 <= 0x3f
where x0 is the most significant byte of x, is emitted where the
constant is smaller and more likely to materialize as an immediate.
Similarly, comparisons of the form
x >= 0x4000000000000000
can be shortened into x0 >= 0x40.
I'm not entirely sure whether combine is the right place to implement
something like this. In my first try I implemented it in
TARGET_CANONICALIZE_COMPARISON but then thought other targets might
profit from it, too. simplify_context::simplify_relational_operation_1
seems to be the wrong place since code/mode may change. Any opinions?
gcc/ChangeLog:
* combine.cc (simplify_compare_const): Narrow comparison of
memory and constant.
(try_combine): Adapt new function signature.
(simplify_comparison): Adapt new function signature.
gcc/testsuite/ChangeLog:
* gcc.target/s390/cmp-mem-const-1.c: New test.
* gcc.target/s390/cmp-mem-const-2.c: New test.
This does seem more general than we'd want to do in the canonicalization
hook. So thanks for going the extra mile and doing a generic
implementation.
@@ -11987,6 +11988,79 @@ simplify_compare_const (enum rtx_code code,
machine_mode mode,
break;
}
+ /* Narrow non-symmetric comparison of memory and constant as e.g.
+ x0...x7 <= 0x3fffffffffffffff into x0 <= 0x3f where x0 is the most
+ significant byte. Likewise, transform x0...x7 >= 0x4000000000000000 into
+ x0 >= 0x40. */
+ if ((code == LEU || code == LTU || code == GEU || code == GTU)
+ && is_a <scalar_int_mode> (GET_MODE (op0), &int_mode)
+ && MEM_P (op0)
+ && !MEM_VOLATILE_P (op0)
+ && (unsigned HOST_WIDE_INT)const_op > 0xff)
+ {
+ unsigned HOST_WIDE_INT n = (unsigned HOST_WIDE_INT)const_op;
+ enum rtx_code adjusted_code = code;
+
+ /* If the least significant bit is already zero, then adjust the
+ comparison in the hope that we hit cases like
+ op0 <= 0x3ffffdfffffffffe
+ where the adjusted comparison
+ op0 < 0x3ffffdffffffffff
+ can be shortened into
+ op0' < 0x3ffffd. */
+ if (code == LEU && (n & 1) == 0)
+ {
+ ++n;
+ adjusted_code = LTU;
+ }
+ /* or e.g. op0 < 0x4020000000000000 */
+ else if (code == LTU && (n & 1) == 0)
+ {
+ --n;
+ adjusted_code = LEU;
+ }
+ /* or op0 >= 0x4000000000000001 */
+ else if (code == GEU && (n & 1) == 1)
+ {
+ --n;
+ adjusted_code = GTU;
+ }
+ /* or op0 > 0x3fffffffffffffff. */
+ else if (code == GTU && (n & 1) == 1)
+ {
+ ++n;
+ adjusted_code = GEU;
+ }
+
+ scalar_int_mode narrow_mode_iter;
+ bool lower_p = code == LEU || code == LTU;
+ bool greater_p = !lower_p;
+ FOR_EACH_MODE_UNTIL (narrow_mode_iter, int_mode)
+ {
+ unsigned nbits = GET_MODE_PRECISION (int_mode)
+ - GET_MODE_PRECISION (narrow_mode_iter);
+ unsigned HOST_WIDE_INT mask = (HOST_WIDE_INT_1U << nbits) - 1;
+ unsigned HOST_WIDE_INT lower_bits = n & mask;
+ if ((lower_p && lower_bits == mask)
+ || (greater_p && lower_bits == 0))
+ {
+ n >>= nbits;
+ break;
+ }
+ }
+
+ if (narrow_mode_iter < int_mode)
+ {
+ poly_int64 offset = BYTES_BIG_ENDIAN
+ ? 0
+ : GET_MODE_SIZE (int_mode)
+ - GET_MODE_SIZE (narrow_mode_iter);
Go ahead and add some parenthesis here. I'd add one pair around the
whole RHS of that assignment. The '?' and ':' would line up under the
'B' in that case. Similarly add them around the false arm of the
ternary. The '-' will line up under the 'G'.
Going to trust you got the little endian adjustment correct here ;-)
/* Compute some predicates to simplify code below. */
diff --git a/gcc/testsuite/gcc.target/s390/cmp-mem-const-1.c
b/gcc/testsuite/gcc.target/s390/cmp-mem-const-1.c
new file mode 100644
index 00000000000..b90c2a8c224
--- /dev/null
+++ b/gcc/testsuite/gcc.target/s390/cmp-mem-const-1.c
@@ -0,0 +1,99 @@
+/* { dg-do compile { target { lp64 } } } */
+/* { dg-options "-O1 -march=z13 -mzarch" } */
+/* { dg-final { scan-assembler-not {\tclc\t} } } */
+
+int
+ge_8b (unsigned long *x)
+{
+ return *x >= 0x4000000000000000;
+}
Would it be possible to add some debugging output in
simplify_compare_const so that you could search for that debugging
output and make these tests generic?
Jeff
