Add a flag to align all basic blocks in the function.

When debugging performance regressions we often ask ourselves if the regression
that we see is due to poor isel/sched/ra or due to some micro-architetural
problem.  When comparing two code sequences one good way to rule out front-end
bottlenecks (and other the issues) is to force code alignment. This pass adds
a flag that forces the alignment of all of the basic blocks in the program.

llvm-svn: 179353

llvm/lib/CodeGen/MachineBlockPlacement.cpp

@@ -39,6 +39,7 @@
 #include "llvm/CodeGen/MachineLoopInfo.h"
 #include "llvm/CodeGen/MachineModuleInfo.h"
 #include "llvm/Support/Allocator.h"
+#include "llvm/Support/CommandLine.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Target/TargetInstrInfo.h"
 #include "llvm/Target/TargetLowering.h"
@@ -52,6 +53,11 @@ STATISTIC(CondBranchTakenFreq,
 STATISTIC(UncondBranchTakenFreq,
           "Potential frequency of taking unconditional branches");
 
+static cl::opt<unsigned> AlignAllBlock("align-all-blocks",
+                                       cl::desc("Force the alignment of all "
+                                                "blocks in the function."),
+                                       cl::init(0), cl::Hidden);
+
 namespace {
 class BlockChain;
 /// \brief Type for our function-wide basic block -> block chain mapping.
@@ -1083,6 +1089,14 @@ bool MachineBlockPlacement::runOnMachineFunction(MachineFunction &F) {
   TLI = F.getTarget().getTargetLowering();
   assert(BlockToChain.empty());
 
+  if (AlignAllBlock) {
+    // Align all of the blocks in the function to a specific alignment.
+    for (MachineFunction::iterator FI = F.begin(), FE = F.end();
+         FI != FE; ++FI)
+      FI->setAlignment(AlignAllBlock);
+    return true;
+  }
+
   buildCFGChains(F);
 
   BlockToChain.clear();

llvm/test/CodeGen/X86/code_placement_align_all.ll

@@ -0,0 +1,22 @@
+; RUN: llc -march=x86 -align-all-blocks=16 < %s | FileCheck %s
+
+;CHECK: foo
+;CHECK: .align  16, 0x90
+;CHECK: .align  16, 0x90
+;CHECK: .align  16, 0x90
+;CHECK: ret
+define i32 @foo(i32 %t, i32 %l) nounwind readnone ssp uwtable {
+  %1 = icmp eq i32 %t, 0
+  br i1 %1, label %4, label %2
+
+; <label>:2                                       ; preds = %0
+  %3 = add nsw i32 %t, 2
+  ret i32 %3
+
+; <label>:4                                       ; preds = %0
+  %5 = icmp eq i32 %l, 0
+  %. = select i1 %5, i32 0, i32 5
+  ret i32 %.
+}
+
+