[llvm] r221904 - ARM: avoid duplicating branches during constant islands.

[Tim Northover]

Author: tnorthover
Date: Thu Nov 13 11:58:51 2014
New Revision: 221904

URL: http://llvm.org/viewvc/llvm-project?rev=221904&view=rev
Log:
ARM: avoid duplicating branches during constant islands.

We were using a naive heuristic to determine whether a basic block already had
an unconditional branch at the end. This mostly corresponded to reality
(assuming branches got optimised) because there's not much point in a branch to
the next block, but could go wrong.

Added:
    llvm/trunk/test/CodeGen/ARM/constant-islands.ll
Modified:
    llvm/trunk/lib/Target/ARM/ARMConstantIslandPass.cpp

Modified: llvm/trunk/lib/Target/ARM/ARMConstantIslandPass.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/ARM/ARMConstantIslandPass.cpp?rev=221904&r1=221903&r2=221904&view=diff
==============================================================================
--- llvm/trunk/lib/Target/ARM/ARMConstantIslandPass.cpp (original)
+++ llvm/trunk/lib/Target/ARM/ARMConstantIslandPass.cpp Thu Nov 13 11:58:51 2014
@@ -275,6 +275,7 @@ namespace {
 
   private:
     void doInitialPlacement(std::vector<MachineInstr*> &CPEMIs);
+    bool BBHasFallthrough(MachineBasicBlock *MBB);
     CPEntry *findConstPoolEntry(unsigned CPI, const MachineInstr *CPEMI);
     unsigned getCPELogAlign(const MachineInstr *CPEMI);
     void scanFunctionJumpTables();
@@ -566,7 +567,7 @@ ARMConstantIslands::doInitialPlacement(s
 
 /// BBHasFallthrough - Return true if the specified basic block can fallthrough
 /// into the block immediately after it.
-static bool BBHasFallthrough(MachineBasicBlock *MBB) {
+bool ARMConstantIslands::BBHasFallthrough(MachineBasicBlock *MBB) {
   // Get the next machine basic block in the function.
   MachineFunction::iterator MBBI = MBB;
   // Can't fall off end of function.
@@ -574,12 +575,15 @@ static bool BBHasFallthrough(MachineBasi
     return false;
 
   MachineBasicBlock *NextBB = std::next(MBBI);
-  for (MachineBasicBlock::succ_iterator I = MBB->succ_begin(),
-       E = MBB->succ_end(); I != E; ++I)
-    if (*I == NextBB)
-      return true;
+  if (std::find(MBB->succ_begin(), MBB->succ_end(), NextBB) == MBB->succ_end())
+    return false;
 
-  return false;
+  // Try to analyze the end of the block. A potential fallthrough may already
+  // have an unconditional branch for whatever reason.
+  MachineBasicBlock *TBB, *FBB;
+  SmallVector<MachineOperand, 4> Cond;
+  bool TooDifficult = TII->AnalyzeBranch(*MBB, TBB, FBB, Cond);
+  return TooDifficult || FBB == nullptr;
 }
 
 /// findConstPoolEntry - Given the constpool index and CONSTPOOL_ENTRY MI,

Added: llvm/trunk/test/CodeGen/ARM/constant-islands.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/ARM/constant-islands.ll?rev=221904&view=auto
==============================================================================
--- llvm/trunk/test/CodeGen/ARM/constant-islands.ll (added)
+++ llvm/trunk/test/CodeGen/ARM/constant-islands.ll Thu Nov 13 11:58:51 2014
@@ -0,0 +1,25 @@
+; RUN: llc -mtriple=thumbv7-linux-gnueabihf -O0 -fast-isel=0 -o - %s | FileCheck %s
+
+define void @test_no_duplicate_branches(float %in) {
+; CHECK-LABEL: test_no_duplicate_branches:
+; CHECK: vldr {{s[0-9]+}}, [[CONST:\.LCPI[0-9]+_[0-9]+]]
+; CHECK: b .LBB
+; CHECK-NOT: b .LBB
+; CHECK: [[CONST]]:
+; CHECK-NEXT: .long 1150963712
+
+  %tst = fcmp oeq float %in, 1234.5
+
+  %chain = zext i1 %tst to i32
+
+  br i1 %tst, label %true, label %false
+
+true:
+  call i32 @llvm.arm.space(i32 2000, i32 undef)
+  ret void
+
+false:
+  ret void
+}
+
+declare i32 @llvm.arm.space(i32, i32)