[llvm-commits] [llvm] r150439 - in /llvm/trunk: lib/Transforms/Scalar/LoopRotation.cpp test/Transforms/LoopRotate/simplifylatch.ll
Andrew Trick
atrick at apple.com
Mon Feb 13 16:00:24 PST 2012
Author: atrick
Date: Mon Feb 13 18:00:23 2012
New Revision: 150439
URL: http://llvm.org/viewvc/llvm-project?rev=150439&view=rev
Log:
Add simplifyLoopLatch to LoopRotate pass.
This folds a simple loop tail into a loop latch. It covers the common (in fortran) case of postincrement loops. It's a "free" way to expose this type of loop to downstream loop optimizations that bail out on non-canonical loops (getLoopLatch is a heavily used check).
Added:
llvm/trunk/test/Transforms/LoopRotate/simplifylatch.ll
Modified:
llvm/trunk/lib/Transforms/Scalar/LoopRotation.cpp
Modified: llvm/trunk/lib/Transforms/Scalar/LoopRotation.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/LoopRotation.cpp?rev=150439&r1=150438&r2=150439&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Scalar/LoopRotation.cpp (original)
+++ llvm/trunk/lib/Transforms/Scalar/LoopRotation.cpp Mon Feb 13 18:00:23 2012
@@ -19,6 +19,7 @@
#include "llvm/Analysis/LoopPass.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/Analysis/ScalarEvolution.h"
+#include "llvm/Analysis/ValueTracking.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/SSAUpdater.h"
@@ -52,6 +53,7 @@
}
bool runOnLoop(Loop *L, LPPassManager &LPM);
+ void simplifyLoopLatch(Loop *L);
bool rotateLoop(Loop *L);
private:
@@ -73,6 +75,11 @@
bool LoopRotate::runOnLoop(Loop *L, LPPassManager &LPM) {
LI = &getAnalysis<LoopInfo>();
+ // Simplify the loop latch before attempting to rotate the header
+ // upward. Rotation may not be needed if the loop tail can be folded into the
+ // loop exit.
+ simplifyLoopLatch(L);
+
// One loop can be rotated multiple times.
bool MadeChange = false;
while (rotateLoop(L))
@@ -146,6 +153,102 @@
}
}
+/// Determine whether the instructions in this range my be safely and cheaply
+/// speculated. This is not an important enough situation to develop complex
+/// heuristics. We handle a single arithmetic instruction along with any type
+/// conversions.
+static bool shouldSpeculateInstrs(BasicBlock::iterator Begin,
+ BasicBlock::iterator End) {
+ bool seenIncrement = false;
+ for (BasicBlock::iterator I = Begin; I != End; ++I) {
+
+ if (!isSafeToSpeculativelyExecute(I))
+ return false;
+
+ if (isa<DbgInfoIntrinsic>(I))
+ continue;
+
+ switch (I->getOpcode()) {
+ default:
+ return false;
+ case Instruction::GetElementPtr:
+ // GEPs are cheap if all indices are constant.
+ if (!cast<GEPOperator>(I)->hasAllConstantIndices())
+ return false;
+ // fall-thru to increment case
+ case Instruction::Add:
+ case Instruction::Sub:
+ case Instruction::And:
+ case Instruction::Or:
+ case Instruction::Xor:
+ case Instruction::Shl:
+ case Instruction::LShr:
+ case Instruction::AShr:
+ if (seenIncrement)
+ return false;
+ seenIncrement = true;
+ break;
+ case Instruction::Trunc:
+ case Instruction::ZExt:
+ case Instruction::SExt:
+ // ignore type conversions
+ break;
+ }
+ }
+ return true;
+}
+
+/// Fold the loop tail into the loop exit by speculating the loop tail
+/// instructions. Typically, this is a single post-increment. In the case of a
+/// simple 2-block loop, hoisting the increment can be much better than
+/// duplicating the entire loop header. In the cast of loops with early exits,
+/// rotation will not work anyway, but simplifyLoopLatch will put the loop in
+/// canonical form so downstream passes can handle it.
+///
+/// I don't believe this invalidates SCEV.
+void LoopRotate::simplifyLoopLatch(Loop *L) {
+ BasicBlock *Latch = L->getLoopLatch();
+ if (!Latch || Latch->hasAddressTaken())
+ return;
+
+ BranchInst *Jmp = dyn_cast<BranchInst>(Latch->getTerminator());
+ if (!Jmp || !Jmp->isUnconditional())
+ return;
+
+ BasicBlock *LastExit = Latch->getSinglePredecessor();
+ if (!LastExit || !L->isLoopExiting(LastExit))
+ return;
+
+ BranchInst *BI = dyn_cast<BranchInst>(LastExit->getTerminator());
+ if (!BI)
+ return;
+
+ if (!shouldSpeculateInstrs(Latch->begin(), Jmp))
+ return;
+
+ DEBUG(dbgs() << "Folding loop latch " << Latch->getName() << " into "
+ << LastExit->getName() << "\n");
+
+ // Hoist the instructions from Latch into LastExit.
+ LastExit->getInstList().splice(BI, Latch->getInstList(), Latch->begin(), Jmp);
+
+ unsigned FallThruPath = BI->getSuccessor(0) == Latch ? 0 : 1;
+ BasicBlock *Header = Jmp->getSuccessor(0);
+ assert(Header == L->getHeader() && "expected a backward branch");
+
+ // Remove Latch from the CFG so that LastExit becomes the new Latch.
+ BI->setSuccessor(FallThruPath, Header);
+ Latch->replaceSuccessorsPhiUsesWith(LastExit);
+ Jmp->eraseFromParent();
+
+ // Nuke the Latch block.
+ assert(Latch->empty() && "unable to evacuate Latch");
+ LI->removeBlock(Latch);
+ if (DominatorTree *DT = getAnalysisIfAvailable<DominatorTree>())
+ DT->eraseNode(Latch);
+ Latch->eraseFromParent();
+}
+
/// Rotate loop LP. Return true if the loop is rotated.
bool LoopRotate::rotateLoop(Loop *L) {
// If the loop has only one block then there is not much to rotate.
Added: llvm/trunk/test/Transforms/LoopRotate/simplifylatch.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/LoopRotate/simplifylatch.ll?rev=150439&view=auto
==============================================================================
--- llvm/trunk/test/Transforms/LoopRotate/simplifylatch.ll (added)
+++ llvm/trunk/test/Transforms/LoopRotate/simplifylatch.ll Mon Feb 13 18:00:23 2012
@@ -0,0 +1,39 @@
+; RUN: opt -S < %s -loop-rotate -verify-dom-info -verify-loop-info | FileCheck %s
+; PR2624 unroll multiple exits
+
+ at mode_table = global [4 x i32] zeroinitializer ; <[4 x i32]*> [#uses=1]
+
+; CHECK: @f
+; CHECK-NOT: bb4
+define i8 @f() {
+entry:
+ tail call i32 @fegetround( ) ; <i32>:0 [#uses=1]
+ br label %bb
+
+bb: ; preds = %bb4, %entry
+ %mode.0 = phi i8 [ 0, %entry ], [ %indvar.next, %bb4 ] ; <i8> [#uses=4]
+ zext i8 %mode.0 to i32 ; <i32>:1 [#uses=1]
+ getelementptr [4 x i32]* @mode_table, i32 0, i32 %1 ; <i32*>:2 [#uses=1]
+ load i32* %2, align 4 ; <i32>:3 [#uses=1]
+ icmp eq i32 %3, %0 ; <i1>:4 [#uses=1]
+ br i1 %4, label %bb1, label %bb2
+
+bb1: ; preds = %bb
+ ret i8 %mode.0
+
+bb2: ; preds = %bb
+ icmp eq i8 %mode.0, 1 ; <i1>:5 [#uses=1]
+ br i1 %5, label %bb5, label %bb4
+
+bb4: ; preds = %bb2
+ %indvar.next = add i8 %mode.0, 1 ; <i8> [#uses=1]
+ br label %bb
+
+bb5: ; preds = %bb2
+ tail call void @raise_exception( ) noreturn
+ unreachable
+}
+
+declare i32 @fegetround()
+
+declare void @raise_exception() noreturn
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