Skip top 64-bit write in movi if identical.

The MacroAssembler's Movi() can synthesise any 128-bit immediate by
splitting it into two 64-bit writes:

    movi vd.2D, #low64
    mov x16, #high64
    ins vd.2D[1], x16

However, if the low and top 64-bits are identical, we can skip the
second write.

Change-Id: I4e8883b20f229647e974e6a693287d17489f7f58

2 files changed, 15 insertions, 4 deletions

diff --git a/src/aarch64/macro-assembler-aarch64.cc b/src/aarch64/macro-assembler-aarch64.cc
index 8574ed7f..ecfdd4d1 100644
--- a/src/aarch64/macro-assembler-aarch64.cc
+++ b/src/aarch64/macro-assembler-aarch64.cc
@@ -1097,11 +1097,15 @@ void MacroAssembler::Movi(const VRegister& vd,
 void MacroAssembler::Movi(const VRegister& vd, uint64_t hi, uint64_t lo) {
   // TODO: Move 128-bit values in a more efficient way.
   VIXL_ASSERT(vd.Is128Bits());
-  UseScratchRegisterScope temps(this);
   Movi(vd.V2D(), lo);
-  Register temp = temps.AcquireX();
-  Mov(temp, hi);
-  Ins(vd.V2D(), 1, temp);
+  if (hi != lo) {
+    UseScratchRegisterScope temps(this);
+    // TODO: Figure out if using a temporary V register to materialise the
+    // immediate is better.
+    Register temp = temps.AcquireX();
+    Mov(temp, hi);
+    Ins(vd.V2D(), 1, temp);
+  }
 }
 
 
diff --git a/test/aarch64/test-disasm-neon-aarch64.cc b/test/aarch64/test-disasm-neon-aarch64.cc
index a8e91e95..e09ebd95 100644
--- a/test/aarch64/test-disasm-neon-aarch64.cc
+++ b/test/aarch64/test-disasm-neon-aarch64.cc
@@ -3099,6 +3099,13 @@ TEST(neon_modimm) {
   COMPARE_MACRO(Movi(v1.V2D(), 0xffff0000ffffff),
                 "movi v1.2d, #0xffff0000ffffff");
 
+  COMPARE_MACRO(Movi(v2.V2D(), 0xff00ff00ff00ff, 0xff00ff00ff00ff),
+                "movi v2.2d, #0xff00ff00ff00ff");
+  COMPARE_MACRO(Movi(v3.V2D(), 0xffff, 0xff00ff00ff00ff),
+                "movi v3.2d, #0xff00ff00ff00ff\n"
+                "mov x16, #0xffff\n"
+                "mov v3.d[1], x16");
+
   COMPARE_MACRO(Fmov(v0.V2S(), 1.0f), "fmov v0.2s, #0x70 (1.0000)");
   COMPARE_MACRO(Fmov(v31.V2S(), -13.0f), "fmov v31.2s, #0xaa (-13.0000)");
   COMPARE_MACRO(Fmov(v0.V4S(), 1.0f), "fmov v0.4s, #0x70 (1.0000)");