Add command line options to force function/loop alignments.

[mcrosier]

Summary

In short, these options can be used to override the target specified preferred/minimum function/loop alignments for testing purposes.

Chad

[gberry]

lib/CodeGen/MachineBlockPlacement.cpp

1344

If you move the max up before the if, you don't need to change the !Align check.

[mcrosier]

Address Geoff's comment.

[mcrosier]

A more straight forward solution.

[gberry]

LGTM with one caveat: we should double-check that the MachineBlock emission handles the case where the padding instruction bytes could be executed (i.e. that it knows to emit nops instead of just zeros).

[mcrosier]

In D15648#315796, @gberry wrote:

LGTM with one caveat: we should double-check that the MachineBlock emission handles the case where the padding instruction bytes could be executed (i.e. that it knows to emit nops instead of just zeros).

I didn't run into any issues across Spec200X. LGTM, per Geoff.

[mcrosier]

Committed r256571.

lib/CodeGen/MachineBlockPlacement.cpp

 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);
 
+static cl::opt<unsigned>
+    AlignAllLoops("align-all-loops",
+                  cl::desc("Force the alignment of all loops in the function."),
+                  cl::init(0), cl::Hidden);
+
 // FIXME: Find a good default for this flag and remove the flag.
 static cl::opt<unsigned> ExitBlockBias(
     "block-placement-exit-block-bias",
     cl::desc("Block frequency percentage a loop exit block needs "
              "over the original exit to be considered the new exit."),
     cl::init(0), cl::Hidden);
 
 static cl::opt<bool> OutlineOptionalBranches(
for (MachineBasicBlock *ChainBB : FunctionChain) {
     // Don't align non-looping basic blocks. These are unlikely to execute
     // enough times to matter in practice. Note that we'll still handle
     // unnatural CFGs inside of a natural outer loop (the common case) and
     // rotated loops.
     MachineLoop *L = MLI->getLoopFor(ChainBB);
     if (!L)
       continue;
 
+    if (AlignAllLoops) {
+      ChainBB->setAlignment(AlignAllLoops);
+      continue;
+    }
+
     unsigned Align = TLI->getPrefLoopAlignment(L);
     if (!Align)
       continue; // Don't care about loop alignment.
 
     // If the block is cold relative to the function entry don't waste space
     // aligning it.
     BlockFrequency Freq = MBFI->getBlockFreq(ChainBB);
     if (Freq < WeightedEntryFreq)
       continue;
 
     // If the block is cold relative to its loop header, don't align it

lib/CodeGen/MachineFunction.cpp

 #include "llvm/Target/TargetFrameLowering.h"
 #include "llvm/Target/TargetLowering.h"
 #include "llvm/Target/TargetMachine.h"
 #include "llvm/Target/TargetSubtargetInfo.h"
 using namespace llvm;
 
 #define DEBUG_TYPE "codegen"
 
+static cl::opt<unsigned>
+    AlignAllFunctions("align-all-functions",
+                      cl::desc("Force the alignment of all functions."),
+                      cl::init(0), cl::Hidden);
+
 void MachineFunctionInitializer::anchor() {}
 
 //===----------------------------------------------------------------------===//
 // MachineFunction implementation
 //===----------------------------------------------------------------------===//
 
 // Out-of-line virtual method.
 MachineFunctionInfo::~MachineFunctionInfo() {}
MachineFunction::MachineFunction(const Function *F, const TargetMachine &TM,
   Alignment = STI->getTargetLowering()->getMinFunctionAlignment();
 
   // FIXME: Shouldn't use pref alignment if explicit alignment is set on Fn.
   // FIXME: Use Function::optForSize().
   if (!Fn->hasFnAttribute(Attribute::OptimizeForSize))
     Alignment = std::max(Alignment,
                          STI->getTargetLowering()->getPrefFunctionAlignment());
 
+  if (AlignAllFunctions)
+    Alignment = AlignAllFunctions;
+
   FunctionNumber = FunctionNum;
   JumpTableInfo = nullptr;
 
   if (isFuncletEHPersonality(classifyEHPersonality(
           F->hasPersonalityFn() ? F->getPersonalityFn() : nullptr))) {
     WinEHInfo = new (Allocator) WinEHFuncInfo();
   }