[aarch64-port-dev ] RFD: scratch registers cleanup [Long post]

[Andrew Haley]

One other thing: this exercise has shown that in many cases we trash
scratch registers in places where it really doesn't matter, and we'd
be much better off rewriting them not to do so.

This makes push_call_clobbered_registers() something that can safely
be used everywhere. But I'm holding off any of this because I want the
first patch to be, if at all possible, neutral with regard to code
generated.

diff -r 33f9271b3167 src/hotspot/cpu/aarch64/macroAssembler_aarch64.cpp
--- a/src/hotspot/cpu/aarch64/macroAssembler_aarch64.cpp	Mon Nov 04 13:13:34 2019 -0500
+++ b/src/hotspot/cpu/aarch64/macroAssembler_aarch64.cpp	Wed Nov 06 08:36:08 2019 -0500
@@ -2624,15 +2624,17 @@
   int step = 4 * wordSize;
   push(RegSet::range(r0, r18) - RegSet::of(rscratch1, rscratch2), sp);
   sub(sp, sp, step);
-  mov(rscratch1, -step);
+  mov(r0, -step);
   // Push v0-v7, v16-v31.
   for (int i = 31; i>= 4; i -= 4) {
     if (i <= v7->encoding() || i >= v16->encoding())
       st1(as_FloatRegister(i-3), as_FloatRegister(i-2), as_FloatRegister(i-1),
-          as_FloatRegister(i), T1D, Address(post(sp, rscratch1)));
+          as_FloatRegister(i), T1D, Address(post(sp, r0)));
   }
   st1(as_FloatRegister(0), as_FloatRegister(1), as_FloatRegister(2),
       as_FloatRegister(3), T1D, Address(sp));
+  // Reload r0 from where it was saved before pushing v0-v7, v16-v31.
+  ldr(r0, Address(sp, (8 + 16) * wordSize));
 }

-- 
Andrew Haley  (he/him)
Java Platform Lead Engineer
Red Hat UK Ltd. <https://www.redhat.com>
https://keybase.io/andrewhaley
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