Bug 1763054 - adjust the encoding of arm64 fp constants. r=nbp

Following Clang11, encode a 0.0 as MOVI and a -0.0 as MOVI+FNEG.  This
avoids gpr->fpr moves and loads from memory everywhere and probably
insulates us a little from microarchitectural differences.

Differential Revision: https://phabricator.services.mozilla.com/D143018

js/src/jit-test/tests/wasm/binop-arm64-ion-codegen.js

@@ -298,7 +298,7 @@ codegenTestARM64_adhoc(
        (func (export "f") (param $a f64) (param $b f64) (param $c f64) (param $d f64) (result f64)
          (select (local.get $b) (local.get $d) (f64.lt (f64.const 0) (local.get $c)))))`,
     'f',
-    `9e6703e0  fmov    d0, xzr
+    `2f00e400  movi    d0, #0x0
      1e622000  fcmp    d0, d2
      1e633c20  fcsel   d0, d1, d3, lo`)
 

js/src/jit/arm64/MacroAssembler-arm64.h

@@ -1585,14 +1585,31 @@ class MacroAssemblerCompat : public vixl::MacroAssembler {
   }
 
   void loadConstantDouble(double d, FloatRegister dest) {
-    // Note, for -0.0 this will turn into a pc-relative load from memory, not a
-    // `fmov + fneg` sequence.  The former is believed to be better, as gcc,
-    // clang, and vixl all choose it.
-    Fmov(ARMFPRegister(dest, 64), d);
+    ARMFPRegister r(dest, 64);
+    if (d == 0.0) {
+      // Clang11 does movi for 0 and movi+fneg for -0, and this seems like a
+      // good implementation-independent strategy as it avoids any gpr->fpr
+      // moves or memory traffic.
+      Movi(r, 0);
+      if (std::signbit(d)) {
+        Fneg(r, r);
+      }
+    } else {
+      Fmov(r, d);
+    }
   }
   void loadConstantFloat32(float f, FloatRegister dest) {
-    // See comment in loadConstantDouble.
-    Fmov(ARMFPRegister(dest, 32), f);
+    ARMFPRegister r(dest, 32);
+    if (f == 0.0) {
+      // See comments above.  There's not a movi variant for a single register,
+      // so clear the double.
+      Movi(ARMFPRegister(dest, 64), 0);
+      if (std::signbit(f)) {
+        Fneg(r, r);
+      }
+    } else {
+      Fmov(r, f);
+    }
   }
 
   void cmpTag(Register tag, ImmTag ref) {