[RFC]rs6000: split complicated constant to memory

[Jiufu Guo <guojiufu@linux.ibm.com>]

Hi,

This patch tries to put the constant into constant pool if building the
constant requires 3 or more instructions.

But there is a concern: I'm wondering if this patch is really profitable.

Because, as I tested, 1. for simple case, if instructions are not been run
in parallel, loading constant from memory maybe faster; but 2. if there
are some instructions could run in parallel, loading constant from memory
are not win comparing with building constant.  As below examples.

For f1.c and f3.c, 'loading' constant would be acceptable in runtime aspect;
for f2.c and f4.c, 'loading' constant are visibly slower. 

For real-world cases, both kinds of code sequences exist.

So, I'm not sure if we need to push this patch.

Run a lot of times (1000000000) below functions to check runtime.
f1.c:
long foo (long *arg, long*, long *)
{
  *arg = 0x1234567800000000;
}
asm building constant:
	lis 10,0x1234
	ori 10,10,0x5678
	sldi 10,10,32
vs.  asm loading
	addis 10,2,.LC0@toc@ha
	ld 10,.LC0@toc@l(10)
The runtime between 'building' and 'loading' are similar: some times the
'building' is faster; sometimes 'loading' is faster. And the difference is
slight.

f2.c
long foo (long *arg, long *arg2, long *arg3)
{
  *arg = 0x1234567800000000;
  *arg2 = 0x7965234700000000;
  *arg3 = 0x4689123700000000;
}
asm building constant:
	lis 7,0x1234
	lis 10,0x7965
	lis 9,0x4689
	ori 7,7,0x5678
	ori 10,10,0x2347
	ori 9,9,0x1237
	sldi 7,7,32
	sldi 10,10,32
	sldi 9,9,32
vs. loading
	addis 7,2,.LC0@toc@ha
	addis 10,2,.LC1@toc@ha
	addis 9,2,.LC2@toc@ha
	ld 7,.LC0@toc@l(7)
	ld 10,.LC1@toc@l(10)
	ld 9,.LC2@toc@l(9)
For this case, 'loading' is always slower than 'building' (>15%).

f3.c
long foo (long *arg, long *, long *)
{
  *arg = 384307168202282325;
}
	lis 10,0x555
	ori 10,10,0x5555
	sldi 10,10,32
	oris 10,10,0x5555
	ori 10,10,0x5555
For this case, 'building' (through 5 instructions) are slower, and 'loading'
is faster ~5%;

f4.c
long foo (long *arg, long *arg2, long *arg3)
{
  *arg = 384307168202282325;
  *arg2 = -6148914691236517205;
  *arg3 = 768614336404564651;
}
	lis 7,0x555
	lis 10,0xaaaa
	lis 9,0xaaa
	ori 7,7,0x5555
	ori 10,10,0xaaaa
	ori 9,9,0xaaaa
	sldi 7,7,32
	sldi 10,10,32
	sldi 9,9,32
	oris 7,7,0x5555
	oris 10,10,0xaaaa
	oris 9,9,0xaaaa
	ori 7,7,0x5555
	ori 10,10,0xaaab
	ori 9,9,0xaaab
For this cases, since 'building' constant are parallel, 'loading' is slower:
~8%. On p10, 'loading'(through 'pld') is also slower >4%.


BR,
Jeff(Jiufu)

---
 gcc/config/rs6000/rs6000.cc                | 14 ++++++++++++++
 gcc/testsuite/gcc.target/powerpc/pr63281.c | 11 +++++++++++
 2 files changed, 25 insertions(+)
 create mode 100644 gcc/testsuite/gcc.target/powerpc/pr63281.c

diff --git a/gcc/config/rs6000/rs6000.cc b/gcc/config/rs6000/rs6000.cc
index 4b727d2a500..3798e11bdbc 100644
--- a/gcc/config/rs6000/rs6000.cc
+++ b/gcc/config/rs6000/rs6000.cc
@@ -10098,6 +10098,20 @@ rs6000_emit_set_const (rtx dest, rtx source)
 	  c = ((c & 0xffffffff) ^ 0x80000000) - 0x80000000;
 	  emit_move_insn (lo, GEN_INT (c));
 	}
+      else if (base_reg_operand (dest, mode)
+	       && num_insns_constant (source, mode) > 2)
+	{
+	  rtx sym = force_const_mem (mode, source);
+	  if (TARGET_TOC && SYMBOL_REF_P (XEXP (sym, 0))
+	      && use_toc_relative_ref (XEXP (sym, 0), mode))
+	    {
+	      rtx toc = create_TOC_reference (XEXP (sym, 0), copy_rtx (dest));
+	      sym = gen_const_mem (mode, toc);
+	      set_mem_alias_set (sym, get_TOC_alias_set ());
+	    }
+
+	  emit_insn (gen_rtx_SET (dest, sym));
+	}
       else
 	rs6000_emit_set_long_const (dest, c);
       break;
diff --git a/gcc/testsuite/gcc.target/powerpc/pr63281.c b/gcc/testsuite/gcc.target/powerpc/pr63281.c
new file mode 100644
index 00000000000..469a8f64400
--- /dev/null
+++ b/gcc/testsuite/gcc.target/powerpc/pr63281.c
@@ -0,0 +1,11 @@
+/* PR target/63281 */
+/* { dg-do compile { target lp64 } } */
+/* { dg-options "-O2 -std=c99" } */
+
+void
+foo (unsigned long long *a)
+{
+  *a = 0x020805006106003;
+}
+
+/* { dg-final { scan-assembler-times {\mp?ld\M} 1 } } */
-- 
2.17.1