[PR16882] Ignore noreturn definitions when setting isPhysRegUsed.

PEI inserts a save/restore sequence for the link register, according to the
information it gets from the MachineRegisterInfo.
MachineRegisterInfo is populated by the VirtRegMap pass.
This pass was not aware of noreturn calls and was registering the definitions of
these calls the same way as regular operations.

Modify VirtRegPass so that it does not set the isPhysRegUsed information for
registers only defined by noreturn calls.
The rational is that a noreturn call is the "last instruction" of the program
(if it returns the behavior is undefined), so everything that is defined by it
cannot be used and will not interfere with anything else. Therefore, it is
pointless to account for then.

llvm-svn: 191349

llvm/lib/CodeGen/VirtRegMap.cpp

@@ -28,6 +28,7 @@
 #include "llvm/CodeGen/MachineInstrBuilder.h"
 #include "llvm/CodeGen/MachineRegisterInfo.h"
 #include "llvm/CodeGen/Passes.h"
+#include "llvm/IR/Function.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/Compiler.h"
 #include "llvm/Support/Debug.h"
@@ -264,15 +265,32 @@ void VirtRegRewriter::rewrite() {
   SmallVector<unsigned, 8> SuperDeads;
   SmallVector<unsigned, 8> SuperDefs;
   SmallVector<unsigned, 8> SuperKills;
+  SmallPtrSet<const MachineInstr *, 4> NoReturnInsts;
 
   for (MachineFunction::iterator MBBI = MF->begin(), MBBE = MF->end();
        MBBI != MBBE; ++MBBI) {
     DEBUG(MBBI->print(dbgs(), Indexes));
+    bool IsExitBB = MBBI->succ_empty();
     for (MachineBasicBlock::instr_iterator
            MII = MBBI->instr_begin(), MIE = MBBI->instr_end(); MII != MIE;) {
       MachineInstr *MI = MII;
       ++MII;
 
+      // Check if this instruction is a call to a noreturn function.
+      // If so, all the definitions set by this instruction can be ignored.
+      if (IsExitBB && MI->isCall())
+        for (MachineInstr::mop_iterator MOI = MI->operands_begin(),
+               MOE = MI->operands_end(); MOI != MOE; ++MOI) {
+          MachineOperand &MO = *MOI;
+          if (!MO.isGlobal())
+            continue;
+          const Function *Func = dyn_cast<Function>(MO.getGlobal());
+          if (!Func || !Func->hasFnAttribute(Attribute::NoReturn))
+            continue;
+          NoReturnInsts.insert(MI);
+          break;
+        }
+
       for (MachineInstr::mop_iterator MOI = MI->operands_begin(),
            MOE = MI->operands_end(); MOI != MOE; ++MOI) {
         MachineOperand &MO = *MOI;
@@ -353,7 +371,25 @@ void VirtRegRewriter::rewrite() {
   }
 
   // Tell MRI about physical registers in use.
-  for (unsigned Reg = 1, RegE = TRI->getNumRegs(); Reg != RegE; ++Reg)
-    if (!MRI->reg_nodbg_empty(Reg))
-      MRI->setPhysRegUsed(Reg);
+  if (NoReturnInsts.empty()) {
+    for (unsigned Reg = 1, RegE = TRI->getNumRegs(); Reg != RegE; ++Reg)
+      if (!MRI->reg_nodbg_empty(Reg))
+        MRI->setPhysRegUsed(Reg);
+  } else {
+    for (unsigned Reg = 1, RegE = TRI->getNumRegs(); Reg != RegE; ++Reg) {
+      if (MRI->reg_nodbg_empty(Reg))
+        continue;
+      // Check if this register has a use that will impact the rest of the
+      // code. Uses in debug and noreturn instructions do not impact the
+      // generated code.
+      for (MachineRegisterInfo::reg_nodbg_iterator It =
+             MRI->reg_nodbg_begin(Reg),
+             EndIt = MRI->reg_nodbg_end(); It != EndIt; ++It) {
+        if (!NoReturnInsts.count(&(*It))) {
+          MRI->setPhysRegUsed(Reg);
+          break;
+        }
+      }
+    }
+  }
 }

llvm/test/CodeGen/ARM/noreturn.ll

@@ -0,0 +1,38 @@
+; RUN: llc -O3 -o - %s | FileCheck %s
+; Test case from PR16882.
+target triple = "thumbv7s-apple-ios"
+
+; Function Attrs: noreturn
+define i32 @test1() #0 {
+; CHECK-LABEL: @test1
+; CHECK-NOT: push
+entry:
+  tail call void @overflow() #0
+  unreachable
+}
+
+; Function Attrs: noreturn
+declare void @overflow() #0
+
+define i32 @test2(i32 %x, i32 %y) {
+; CHECK-LABEL: @test2
+; CHECK-NOT: push
+; CHECK-NOT: pop
+entry:
+  %conv = sext i32 %x to i64
+  %conv1 = sext i32 %y to i64
+  %mul = mul nsw i64 %conv1, %conv
+  %conv2 = trunc i64 %mul to i32
+  %conv3 = sext i32 %conv2 to i64
+  %cmp = icmp eq i64 %mul, %conv3
+  br i1 %cmp, label %if.end, label %if.then
+
+if.then:                                          ; preds = %entry
+  tail call void @overflow() #0
+  unreachable
+
+if.end:                                           ; preds = %entry
+  ret i32 %conv2
+}
+
+attributes #0 = { noreturn }