Fix MachineSink to be able to sink instructions that use physical registers

which have no defs anywhere in the function. In particular, this fixes sinking
of instructions that reference RIP on x86-64, which is currently being modeled
as a register.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@82815 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 32 insertions, 1 deletions

diff --git a/lib/CodeGen/MachineSink.cpp b/lib/CodeGen/MachineSink.cpp
index f975ad344a..5f555b2f63 100644
--- a/lib/CodeGen/MachineSink.cpp
+++ b/lib/CodeGen/MachineSink.cpp
@@ -35,6 +35,7 @@ namespace {
   class VISIBILITY_HIDDEN MachineSinking : public MachineFunctionPass {
     const TargetMachine   *TM;
     const TargetInstrInfo *TII;
+    const TargetRegisterInfo *TRI;
     MachineFunction       *CurMF; // Current MachineFunction
     MachineRegisterInfo  *RegInfo; // Machine register information
     MachineDominatorTree *DT;   // Machine dominator tree
@@ -95,6 +96,7 @@ bool MachineSinking::runOnMachineFunction(MachineFunction &MF) {
   CurMF = &MF;
   TM = &CurMF->getTarget();
   TII = TM->getInstrInfo();
+  TRI = TM->getRegisterInfo();
   RegInfo = &CurMF->getRegInfo();
   DT = &getAnalysis<MachineDominatorTree>();
 
@@ -176,8 +178,19 @@ bool MachineSinking::SinkInstruction(MachineInstr *MI, bool &SawStore) {
     if (TargetRegisterInfo::isPhysicalRegister(Reg)) {
       // If this is a physical register use, we can't move it.  If it is a def,
       // we can move it, but only if the def is dead.
-      if (MO.isUse() || !MO.isDead())
+      if (MO.isUse()) {
+        // If the physreg has no defs anywhere, it's just an ambient register
+        // and we can freely move its uses.
+        if (!RegInfo->def_empty(Reg))
+          return false;
+        // Check for a def among the register's aliases too.
+        for (const unsigned *Alias = TRI->getAliasSet(Reg); *Alias; ++Alias)
+          if (!RegInfo->def_empty(*Alias))
+            return false;
+      } else if (!MO.isDead()) {
+        // A def that isn't dead. We can't move it.
         return false;
+      }
     } else {
       // Virtual register uses are always safe to sink.
       if (MO.isUse()) continue;
diff --git a/test/CodeGen/X86/sink.ll b/test/CodeGen/X86/sink.ll
new file mode 100644
index 0000000000..1d128b9006
--- /dev/null
+++ b/test/CodeGen/X86/sink.ll
@@ -0,0 +1,18 @@
+; RUN: llc < %s -march=x86-64 -asm-verbose=false | FileCheck %s
+
+; Currently, floating-point selects are lowered to CFG triangles.
+; This means that one side of the select is always unconditionally
+; evaluated, however with MachineSink we can sink the other side so
+; that it's conditionally evaluated.
+
+; CHECK: foo:
+; CHECK-NEXT: divsd
+; CHECK-NEXT: testb $1, %dil
+; CHECK-NEXT: jne
+
+define double @foo(double %x, double %y, i1 %c) nounwind {
+  %a = fdiv double %x, 3.2
+  %b = fdiv double %y, 3.3
+  %z = select i1 %c, double %a, double %b
+  ret double %z
+}