[llvm] [X86][GlobalISel] Added support for llvm.get.rounding (PR #147716)

[Evgenii Kudriashov via llvm-commits]

================
@@ -777,6 +782,82 @@ bool X86LegalizerInfo::legalizeNarrowingStore(MachineInstr &MI,
   return true;
 }
 
+bool X86LegalizerInfo::legalizeGETROUNDING(MachineInstr &MI,
+                                           MachineRegisterInfo &MRI,
+                                           LegalizerHelper &Helper) const {
+  /*
+   The rounding mode is in bits 11:10 of FPSR, and has the following
+   settings:
+     00 Round to nearest
+     01 Round to -inf
+     10 Round to +inf
+     11 Round to 0
+
+  GET_ROUNDING, on the other hand, expects the following:
+    -1 Undefined
+     0 Round to 0
+     1 Round to nearest
+     2 Round to +inf
+     3 Round to -inf
+
+  To perform the conversion, we use a packed lookup table of the four 2-bit
+  values that we can index by FPSP[11:10]
+    0x2d --> (0b00,10,11,01) --> (0,2,3,1) >> FPSR[11:10]
+
+    (0x2d >> ((FPSR >> 9) & 6)) & 3
+  */
+
+  MachineIRBuilder &MIRBuilder = Helper.MIRBuilder;
+  MachineFunction &MF = MIRBuilder.getMF();
+  Register Dst = MI.getOperand(0).getReg();
+  LLT DstTy = MRI.getType(Dst);
+  const LLT s8 = LLT::scalar(8);
+  const LLT s16 = LLT::scalar(16);
+  const LLT s32 = LLT::scalar(32);
+
+  // Save FP Control Word to stack slot
+  int MemSize = 2;
+  Align Alignment = Align(2);
+  MachinePointerInfo PtrInfo;
+  auto StackTemp = Helper.createStackTemporary(TypeSize::getFixed(MemSize),
+                                               Alignment, PtrInfo);
+  Register StackPtr = StackTemp.getReg(0);
----------------
e-kud wrote:

```suggestion
  Register StackPtr = Helper.createStackTemporary(TypeSize::getFixed(MemSize),
                                                  Alignment, PtrInfo).getReg(0);
```
I don't see any `StackTemp` usage.

https://github.com/llvm/llvm-project/pull/147716