Teach branch folding pass about implicit_def instructions. Unfortunately we can't just eliminate them since register scavenger expects every register use to be defined. However, we can delete them when there are no intra-block uses. Carefully removing some implicit def's which enable more blocks to be optimized away.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@49461 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/BranchFolding.cpp

@@ -27,6 +27,7 @@
 #include "llvm/Target/TargetRegisterInfo.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/Debug.h"
+#include "llvm/ADT/SmallSet.h"
 #include "llvm/ADT/Statistic.h"
 #include "llvm/ADT/STLExtras.h"
 #include <algorithm>
@@ -78,6 +79,7 @@ namespace {
     bool OptimizeBranches(MachineFunction &MF);
     void OptimizeBlock(MachineBasicBlock *MBB);
     void RemoveDeadBlock(MachineBasicBlock *MBB);
+    bool OptimizeImpDefsBlock(MachineBasicBlock *MBB);
     
     bool CanFallThrough(MachineBasicBlock *CurBB);
     bool CanFallThrough(MachineBasicBlock *CurBB, bool BranchUnAnalyzable,
@@ -117,10 +119,63 @@ void BranchFolder::RemoveDeadBlock(MachineBasicBlock *MBB) {
   MF->getBasicBlockList().erase(MBB);
 }
 
+/// OptimizeImpDefsBlock - If a basic block is just a bunch of implicit_def
+/// followed by terminators, and if the implicitly defined registers are not
+/// used by the terminators, remove those implicit_def's. e.g.
+/// BB1:
+///   r0 = implicit_def
+///   r1 = implicit_def
+///   br
+/// This block can be optimized away later if the implicit instructions are
+/// removed.
+bool BranchFolder::OptimizeImpDefsBlock(MachineBasicBlock *MBB) {
+  SmallSet<unsigned, 4> ImpDefRegs;
+  MachineBasicBlock::iterator I = MBB->begin();
+  while (I != MBB->end()) {
+    if (I->getOpcode() != TargetInstrInfo::IMPLICIT_DEF)
+      break;
+    unsigned Reg = I->getOperand(0).getReg();
+    ImpDefRegs.insert(Reg);
+    for (const unsigned *SubRegs = RegInfo->getSubRegisters(Reg);
+         unsigned SubReg = *SubRegs; ++SubRegs)
+      ImpDefRegs.insert(SubReg);
+    ++I;
+  }
+  if (ImpDefRegs.empty())
+    return false;
+
+  MachineBasicBlock::iterator FirstTerm = I;
+  while (I != MBB->end()) {
+    if (!TII->isUnpredicatedTerminator(I))
+      return false;
+    // See if it uses any of the implicitly defined registers.
+    for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) {
+      MachineOperand &MO = I->getOperand(i);
+      if (!MO.isReg() || !MO.isUse())
+        continue;
+      unsigned Reg = MO.getReg();
+      if (ImpDefRegs.count(Reg))
+        return false;
+    }
+    ++I;
+  }
+
+  I = MBB->begin();
+  while (I != FirstTerm) {
+    MachineInstr *ImpDefMI = &*I;
+    ++I;
+    MBB->erase(ImpDefMI);
+  }
+
+  return true;
+}
+
 bool BranchFolder::runOnMachineFunction(MachineFunction &MF) {
   TII = MF.getTarget().getInstrInfo();
   if (!TII) return false;
 
+  RegInfo = MF.getTarget().getRegisterInfo();
+
   // Fix CFG.  The later algorithms expect it to be right.
   bool EverMadeChange = false;
   for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; I++) {
@@ -128,9 +183,9 @@ bool BranchFolder::runOnMachineFunction(MachineFunction &MF) {
     std::vector<MachineOperand> Cond;
     if (!TII->AnalyzeBranch(*MBB, TBB, FBB, Cond))
       EverMadeChange |= MBB->CorrectExtraCFGEdges(TBB, FBB, !Cond.empty());
+    EverMadeChange |= OptimizeImpDefsBlock(MBB);
   }
 
-  RegInfo = MF.getTarget().getRegisterInfo();
   RS = RegInfo->requiresRegisterScavenging(MF) ? new RegScavenger() : NULL;
 
   MMI = getAnalysisToUpdate<MachineModuleInfo>();

test/CodeGen/X86/2007-06-14-branchfold.ll

@@ -1,8 +1,6 @@
-; RUN: llvm-as < %s | llc -mcpu=i686 | not grep jmp
+; RUN: llvm-as < %s | llc -march=x86 -mcpu=i686 | not grep jmp
 ; check that branch folding understands FP_REG_KILL is not a branch
 
-; ModuleID = 'g.bc'
-target datalayout = "e-p:32:32:32-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:32:64-f32:32:32-f64:32:64-v64:64:64-v128:128:128-a0:0:64"
 target triple = "i686-pc-linux-gnu"
   %struct.FRAME.c34003a = type { float, float }
 @report_E = global i8 0   ; <i8*> [#uses=0]

test/CodeGen/X86/2008-04-09-BranchFolding.ll

@@ -0,0 +1,48 @@
+; RUN: llvm-as < %s | llc -march=x86 | not grep jmp
+
+	%struct..0anon = type { i32 }
+	%struct.binding_level = type { %struct.tree_node*, %struct.tree_node*, %struct.tree_node*, %struct.tree_node*, %struct.tree_node*, %struct.binding_level*, i8, i8, i8, i8, i8, i32, %struct.tree_node* }
+	%struct.lang_decl = type opaque
+	%struct.rtx_def = type { i16, i8, i8, [1 x %struct..0anon] }
+	%struct.tree_decl = type { [12 x i8], i8*, i32, %struct.tree_node*, i32, i8, i8, i8, i8, %struct.tree_node*, %struct.tree_node*, %struct.tree_node*, %struct.tree_node*, %struct.tree_node*, %struct.tree_node*, %struct.tree_node*, %struct.tree_node*, %struct.tree_node*, %struct.rtx_def*, %struct..0anon, { %struct.rtx_def* }, %struct.tree_node*, %struct.lang_decl* }
+	%struct.tree_node = type { %struct.tree_decl }
+
+define fastcc %struct.tree_node* @pushdecl(%struct.tree_node* %x) nounwind  {
+entry:
+	%tmp3.i40 = icmp eq %struct.binding_level* null, null		; <i1> [#uses=2]
+	br i1 false, label %bb143, label %bb140
+bb140:		; preds = %entry
+	br i1 %tmp3.i40, label %bb160, label %bb17.i
+bb17.i:		; preds = %bb140
+	ret %struct.tree_node* null
+bb143:		; preds = %entry
+	%tmp8.i43 = load %struct.tree_node** null, align 4		; <%struct.tree_node*> [#uses=1]
+	br i1 %tmp3.i40, label %bb160, label %bb9.i48
+bb9.i48:		; preds = %bb143
+	ret %struct.tree_node* null
+bb160:		; preds = %bb143, %bb140
+	%t.0.reg2mem.0 = phi %struct.tree_node* [ null, %bb140 ], [ %tmp8.i43, %bb143 ]		; <%struct.tree_node*> [#uses=1]
+	%tmp162 = icmp eq %struct.tree_node* %t.0.reg2mem.0, null		; <i1> [#uses=2]
+	br i1 %tmp162, label %bb174, label %bb165
+bb165:		; preds = %bb160
+	br label %bb174
+bb174:		; preds = %bb165, %bb160
+	%line.0 = phi i32 [ 0, %bb165 ], [ undef, %bb160 ]		; <i32> [#uses=1]
+	%file.0 = phi i8* [ null, %bb165 ], [ undef, %bb160 ]		; <i8*> [#uses=1]
+	br i1 %tmp162, label %bb344, label %bb73.i
+bb73.i:		; preds = %bb174
+	br i1 false, label %bb226.i, label %bb220.i
+bb220.i:		; preds = %bb73.i
+	ret %struct.tree_node* null
+bb226.i:		; preds = %bb73.i
+	br i1 false, label %bb260, label %bb273.i
+bb273.i:		; preds = %bb226.i
+	ret %struct.tree_node* null
+bb260:		; preds = %bb226.i
+	tail call void (i8*, i32, ...)* @pedwarn_with_file_and_line( i8* %file.0, i32 %line.0, i8* null ) nounwind 
+	ret %struct.tree_node* null
+bb344:		; preds = %bb174
+	ret %struct.tree_node* null
+}
+
+declare void @pedwarn_with_file_and_line(i8*, i32, ...) nounwind