Reverse order of the two branches at end of a basic block if it is profitable.

We branch to the successor with higher edge weight first.
Convert from
     je    LBB4_8  --> to outer loop
     jmp   LBB4_14 --> to inner loop
to
     jne   LBB4_14
     jmp   LBB4_8

PR12750
rdar: 11393714

llvm-svn: 161018

llvm/lib/CodeGen/MachineBlockPlacement.cpp

@@ -985,8 +985,22 @@ void MachineBlockPlacement::buildCFGChains(MachineFunction &F) {
     // boiler plate.
     Cond.clear();
     MachineBasicBlock *TBB = 0, *FBB = 0; // For AnalyzeBranch.
-    if (!TII->AnalyzeBranch(*PrevBB, TBB, FBB, Cond))
+    if (!TII->AnalyzeBranch(*PrevBB, TBB, FBB, Cond)) {
+      // If PrevBB has a two-way branch, try to re-order the branches
+      // such that we branch to the successor with higher weight first.
+      if (TBB && !Cond.empty() && FBB &&
+          MBPI->getEdgeWeight(PrevBB, FBB) > MBPI->getEdgeWeight(PrevBB, TBB) &&
+          !TII->ReverseBranchCondition(Cond)) {
+        DEBUG(dbgs() << "Reverse order of the two branches: "
+                     << getBlockName(PrevBB) << "\n");
+        DEBUG(dbgs() << "    Edge weight: " << MBPI->getEdgeWeight(PrevBB, FBB)
+                     << " vs " << MBPI->getEdgeWeight(PrevBB, TBB) << "\n");
+        DebugLoc dl;  // FIXME: this is nowhere
+        TII->RemoveBranch(*PrevBB);
+        TII->InsertBranch(*PrevBB, FBB, TBB, Cond, dl);
+      }
       PrevBB->updateTerminator();
+    }
   }
 
   // Fixup the last block.

llvm/test/CodeGen/X86/reverse_branches.ll

@@ -0,0 +1,104 @@
+; RUN: llc -mtriple=x86_64-apple-darwin < %s | FileCheck %s
+
+@.str2 = private unnamed_addr constant [7 x i8] c"memchr\00", align 1
+@.str3 = private unnamed_addr constant [11 x i8] c"bsd_memchr\00", align 1
+@str4 = private unnamed_addr constant [5 x i8] c"Bug!\00"
+
+; Make sure at end of do.cond.i, we jump to do.body.i first to have a tighter
+; inner loop.
+define i32 @test_branches_order() uwtable ssp {
+; CHECK: test_branches_order:
+; CHECK: [[L0:LBB0_[0-9]+]]: ## %do.body.i
+; CHECK: je
+; CHECK: %do.cond.i
+; CHECK: jne [[L0]]
+; CHECK: jmp
+; CHECK: %exit
+entry:
+  %strs = alloca [1000 x [1001 x i8]], align 16
+  br label %for.cond
+
+for.cond:
+  %j.0 = phi i32 [ 0, %entry ], [ %inc10, %for.inc9 ]
+  %cmp = icmp slt i32 %j.0, 1000
+  br i1 %cmp, label %for.cond1, label %for.end11
+
+for.cond1:
+  %indvars.iv50 = phi i64 [ %indvars.iv.next51, %for.body3 ], [ 0, %for.cond ]
+  %0 = trunc i64 %indvars.iv50 to i32
+  %cmp2 = icmp slt i32 %0, 1000
+  br i1 %cmp2, label %for.body3, label %for.inc9
+
+for.body3:
+  %arraydecay = getelementptr inbounds [1000 x [1001 x i8]]* %strs, i64 0, i64 %indvars.iv50, i64 0
+  %call = call i8* @memchr(i8* %arraydecay, i32 120, i64 1000)
+  %add.ptr = getelementptr inbounds [1000 x [1001 x i8]]* %strs, i64 0, i64 %indvars.iv50, i64 %indvars.iv50
+  %cmp7 = icmp eq i8* %call, %add.ptr
+  %indvars.iv.next51 = add i64 %indvars.iv50, 1
+  br i1 %cmp7, label %for.cond1, label %if.then
+
+if.then:
+  %puts = call i32 @puts(i8* getelementptr inbounds ([5 x i8]* @str4, i64 0, i64 0))
+  call void @exit(i32 1) noreturn
+  unreachable
+
+for.inc9:
+  %inc10 = add nsw i32 %j.0, 1
+  br label %for.cond
+
+for.end11:
+  %puts42 = call i32 @puts(i8* getelementptr inbounds ([7 x i8]* @.str2, i64 0, i64 0))
+  br label %for.cond14
+
+for.cond14:
+  %j13.0 = phi i32 [ 0, %for.end11 ], [ %inc39, %for.inc38 ]
+  %cmp15 = icmp slt i32 %j13.0, 1000
+  br i1 %cmp15, label %for.cond18, label %for.end40
+
+for.cond18:
+  %indvars.iv = phi i64 [ %indvars.iv.next, %exit ], [ 0, %for.cond14 ]
+  %1 = trunc i64 %indvars.iv to i32
+  %cmp19 = icmp slt i32 %1, 1000
+  br i1 %cmp19, label %for.body20, label %for.inc38
+
+for.body20:
+  %arraydecay24 = getelementptr inbounds [1000 x [1001 x i8]]* %strs, i64 0, i64 %indvars.iv, i64 0
+  br label %do.body.i
+
+do.body.i:
+  %n.addr.0.i = phi i64 [ %dec.i, %do.cond.i ], [ 1000, %for.body20 ]
+  %p.0.i = phi i8* [ %incdec.ptr.i, %do.cond.i ], [ %arraydecay24, %for.body20 ]
+  %2 = load i8* %p.0.i, align 1
+  %cmp3.i = icmp eq i8 %2, 120
+  br i1 %cmp3.i, label %exit, label %do.cond.i
+
+do.cond.i:
+  %incdec.ptr.i = getelementptr inbounds i8* %p.0.i, i64 1
+  %dec.i = add i64 %n.addr.0.i, -1
+  %cmp5.i = icmp eq i64 %dec.i, 0
+  br i1 %cmp5.i, label %if.then32, label %do.body.i
+
+exit:
+  %add.ptr30 = getelementptr inbounds [1000 x [1001 x i8]]* %strs, i64 0, i64 %indvars.iv, i64 %indvars.iv
+  %cmp31 = icmp eq i8* %p.0.i, %add.ptr30
+  %indvars.iv.next = add i64 %indvars.iv, 1
+  br i1 %cmp31, label %for.cond18, label %if.then32
+
+if.then32:
+  %puts43 = call i32 @puts(i8* getelementptr inbounds ([5 x i8]* @str4, i64 0, i64 0))
+  call void @exit(i32 1) noreturn
+  unreachable
+
+for.inc38:
+  %inc39 = add nsw i32 %j13.0, 1
+  br label %for.cond14
+
+for.end40:
+  %puts44 = call i32 @puts(i8* getelementptr inbounds ([11 x i8]* @.str3, i64 0, i64 0))
+  ret i32 0
+}
+
+declare i8* @memchr(i8*, i32, i64) nounwind readonly
+declare void @exit(i32) noreturn
+declare i32 @puts(i8* nocapture) nounwind
+