There is a functionality as const_anchor in cse.cc. This const_anchor
supports to generate new constants through adding small gap/offsets to
existing constant. For example:
void __attribute__ ((noinline)) foo (long long *a)
{
*a++ = 0x2351847027482577LL;
*a++ = 0x2351847027482578LL;
}
The second constant (0x2351847027482578LL) can be compated by adding '1'
to the first constant (0x2351847027482577LL).
This is profitable if more than one instructions are need to build the
second constant.
* For rs6000, we can enable this functionality, as the instruction
'addi' is just for this when gap is smaller than 0x8000.
* One potential side effect of this feature:
Comparing with
"r101=0x2351847027482577LL
...
r201=0x2351847027482578LL"
The new r201 will be "r201=r101+1", and then r101 will live longer,
and would increase pressure when allocating registers.
But I feel, this would be acceptable for this const_anchor feature.
With this feature, for GCC source code and SPEC object files, the
significant changes are the improvement that: "addi" vs. "2 or more
insns: lis+or.."; it also exposes some other optimizations
opportunities: like combine/jump2. While the side effect is also
occurring in few cases, but it does not impact overall performance.
gcc/ChangeLog:
* config/rs6000/rs6000.cc (TARGET_CONST_ANCHOR): New define.
gcc/testsuite/ChangeLog:
* gcc.target/powerpc/const_anchors.c: New test.
* gcc.target/powerpc/try_const_anchors_ice.c: New test.
#undef TARGET_UPDATE_IPA_FN_TARGET_INFO
#define TARGET_UPDATE_IPA_FN_TARGET_INFO rs6000_update_ipa_fn_target_info
+
+#undef TARGET_CONST_ANCHOR
+#define TARGET_CONST_ANCHOR 0x8000
+
\f
/* Processor table. */
--- /dev/null
+/* { dg-do compile { target has_arch_ppc64 } } */
+/* { dg-options "-O2" } */
+
+#define C1 0x2351847027482577ULL
+#define C2 0x2351847027482578ULL
+
+void __attribute__ ((noinline)) foo (long long *a)
+{
+ *a++ = C1;
+ *a++ = C2;
+}
+
+void __attribute__ ((noinline)) foo1 (long long *a, long long b)
+{
+ *a++ = C1;
+ if (b)
+ *a++ = C2;
+}
+
+/* { dg-final { scan-assembler-times {\maddi\M} 2 } } */
--- /dev/null
+/* { dg-do compile } */
+/* { dg-options "-O2" } */
+
+/* __builtin_stack_restore could generates {[%1:DI]=0;} in BLK mode,
+ it could assert in try_const_anchors which only accepts SCALAR_INT. */
+
+long
+foo (const int val)
+{
+ if (val == (0))
+ return 0;
+ void *p = __builtin_stack_save ();
+ char c = val;
+ __builtin_stack_restore (p);
+ return c;
+}