[SDAG] Preserve unpredictable metadata, teach X86CmovConversion to respect this metadata

Sometimes an developer would like to have more control over cmov vs branch. We have unpredictable metadata in LLVM IR, but currently it is ignored by X86 backend. Propagate this metadata and avoid cmov->branch conversion in X86CmovConversion for cmov with this metadata.

Example:

```
int MaxIndex(int n, int *a) {
    int t = 0;
    for (int i = 1; i < n; i++) {
        // cmov is converted to branch by X86CmovConversion
        if (a[i] > a[t]) t = i;
    }
    return t;
}

int MaxIndex2(int n, int *a) {
    int t = 0;
    for (int i = 1; i < n; i++) {
        // cmov is preserved
        if (__builtin_unpredictable(a[i] > a[t])) t = i;
    }
    return t;
}
```

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D118118

1 files changed, 6 insertions, 3 deletions

diff --git a/llvm/lib/CodeGen/MachineInstr.cpp b/llvm/lib/CodeGen/MachineInstr.cpp
index f296533..400e76f 100644
--- a/llvm/lib/CodeGen/MachineInstr.cpp
+++ b/llvm/lib/CodeGen/MachineInstr.cpp
@@ -534,14 +534,14 @@ void MachineInstr::cloneInstrSymbols(MachineFunction &MF,
   setPCSections(MF, MI.getPCSections());
 }
 
-uint16_t MachineInstr::mergeFlagsWith(const MachineInstr &Other) const {
+uint32_t MachineInstr::mergeFlagsWith(const MachineInstr &Other) const {
   // For now, the just return the union of the flags. If the flags get more
   // complicated over time, we might need more logic here.
   return getFlags() | Other.getFlags();
 }
 
-uint16_t MachineInstr::copyFlagsFromInstruction(const Instruction &I) {
-  uint16_t MIFlags = 0;
+uint32_t MachineInstr::copyFlagsFromInstruction(const Instruction &I) {
+  uint32_t MIFlags = 0;
   // Copy the wrapping flags.
   if (const OverflowingBinaryOperator *OB =
           dyn_cast<OverflowingBinaryOperator>(&I)) {
@@ -575,6 +575,9 @@ uint16_t MachineInstr::copyFlagsFromInstruction(const Instruction &I) {
       MIFlags |= MachineInstr::MIFlag::FmReassoc;
   }
 
+  if (I.getMetadata(LLVMContext::MD_unpredictable))
+    MIFlags |= MachineInstr::MIFlag::Unpredictable;
+
   return MIFlags;
 }