[FastISel][AArch64] Fix a latent bug in floating-point materialization.

The floating-point value positive zero (+0.0) is a valid immedate value
according to isFPImmLegal. As a result AArch64 FastISel went ahead and
used the immediate version of fmov to materialize the constant.

The problem is that the immediate version of fmov cannot encode an imediate for
postive zero. Instead a fmov from the zero register was supposed to be used in
this case.

This fix adds handling for this special case and uses fmov from the zero
register to materialize a positive zero (negative zeroes go to the constant
pool).

There is no test case for this, because this code is currently dead. It will be
enabled in a future commit and I will add a test case in a separate commit
after that.

This fixes <rdar://problem/18027157>.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215753 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Target/AArch64/AArch64FastISel.cpp b/lib/Target/AArch64/AArch64FastISel.cpp
index 603c76f0c26732fed572a0ab4173d6a3f9db85af..5114acaec41afe07512039b3ac00eb1ea9d8e6eb 100644 (file)
--- a/lib/Target/AArch64/AArch64FastISel.cpp
+++ b/lib/Target/AArch64/AArch64FastISel.cpp
@@ -230,10 +230,19 @@ unsigned AArch64FastISel::AArch64MaterializeFP(const ConstantFP *CFP, MVT VT) {
   // This checks to see if we can use FMOV instructions to materialize
   // a constant, otherwise we have to materialize via the constant pool.
   if (TLI.isFPImmLegal(Val, VT)) {
+    unsigned ResultReg = createResultReg(TLI.getRegClassFor(VT));
+    // Positive zero (+0.0) has to be materialized with a fmov from the zero
+    // register, because the immediate version of fmov cannot encode zero.
+    if (Val.isPosZero()) {
+      unsigned ZReg = Is64Bit ? AArch64::XZR : AArch64::WZR;
+      unsigned Opc = Is64Bit ? AArch64::FMOVDr : AArch64::FMOVSr;
+      BuildMI(*FuncInfo.MBB, FuncInfo.InsertPt, DbgLoc, TII.get(Opc), ResultReg)
+        .addReg(ZReg, getKillRegState(true));
+      return ResultReg;
+    }
     int Imm = Is64Bit ? AArch64_AM::getFP64Imm(Val)
                       : AArch64_AM::getFP32Imm(Val);
     unsigned Opc = Is64Bit ? AArch64::FMOVDi : AArch64::FMOVSi;
-    unsigned ResultReg = createResultReg(TLI.getRegClassFor(VT));
     BuildMI(*FuncInfo.MBB, FuncInfo.InsertPt, DbgLoc, TII.get(Opc), ResultReg)
       .addImm(Imm);
     return ResultReg;