[SCEV] Strengthen huge constant trip multiples.

SCEV determines that loops with trip count >=2^32 have a trip multiple
of 1 to guard against huge multiples. This patch stregthens this to
instead find the greatest power of 2 divisor that is less than the
threshold.

Differential Revision: https://reviews.llvm.org/D147868

diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index ce77cce..2831752 100644 (file)
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -8235,22 +8235,25 @@ unsigned ScalarEvolution::getSmallConstantTripMultiple(const Loop *L,
   // Get the trip count
   const SCEV *TCExpr = getTripCountFromExitCount(applyLoopGuards(ExitCount, L));
 
+  // If a trip multiple is huge (>=2^32), the trip count is still divisible by
+  // the greatest power of 2 divisor less than 2^32.
+  auto GetSmallMultiple = [](unsigned TrailingZeros) {
+    return 1U << std::min((uint32_t)31, TrailingZeros);
+  };
+
   const SCEVConstant *TC = dyn_cast<SCEVConstant>(TCExpr);
   if (!TC)
     // Attempt to factor more general cases. Returns the greatest power of
-    // two divisor. If overflow happens, the trip count expression is still
-    // divisible by the greatest power of 2 divisor returned.
-    return 1U << std::min((uint32_t)31, GetMinTrailingZeros(TCExpr));
+    // two divisor.
+    return GetSmallMultiple(GetMinTrailingZeros(TCExpr));
 
   ConstantInt *Result = TC->getValue();
-
-  // Guard against huge trip counts (this requires checking
-  // for zero to handle the case where the trip count == -1 and the
-  // addition wraps).
   assert(Result && "SCEVConstant expected to have non-null ConstantInt");
-  if (Result->getValue().getActiveBits() > 32 ||
-      Result->getValue().getActiveBits() == 0)
-    return 1;
+  assert(Result->getValue() != 0 && "trip count should never be zero");
+
+  // Guard against huge trip multiples.
+  if (Result->getValue().getActiveBits() > 32)
+    return GetSmallMultiple(Result->getValue().countTrailingZeros());
 
   return (unsigned)Result->getZExtValue();
 }

diff --git a/llvm/test/Analysis/ScalarEvolution/huge-trip-multiple.ll b/llvm/test/Analysis/ScalarEvolution/huge-trip-multiple.ll
index d27fc80..f511d04 100644 (file)
--- a/llvm/test/Analysis/ScalarEvolution/huge-trip-multiple.ll
+++ b/llvm/test/Analysis/ScalarEvolution/huge-trip-multiple.ll
@@ -1,7 +1,8 @@
 ; NOTE: Assertions have been autogenerated by utils/update_analyze_test_checks.py UTC_ARGS: --version 2
 ; RUN: opt < %s -disable-output "-passes=print<scalar-evolution>" 2>&1 | FileCheck %s
 
-; Tests loops with huge trip counts. Trip count of >=2**32 are huge. Huge trip counts have a trip multiple of 1.
+; Tests loops with huge trip counts. Trip count of >=2^32 are huge. Huge trip counts have a trip multiple
+; of the greatest power of 2 less than 2^32.
 
 declare void @foo(...)
 
@@ -47,7 +48,7 @@ define void @trip_count_4294967296() {
 ; CHECK-NEXT:  Loop %for.body: symbolic max backedge-taken count is 4294967295
 ; CHECK-NEXT:  Loop %for.body: Predicated backedge-taken count is 4294967295
 ; CHECK-NEXT:   Predicates:
-; CHECK:       Loop %for.body: Trip multiple is 1
+; CHECK:       Loop %for.body: Trip multiple is 2147483648
 ;
 entry:
   br label %for.body
@@ -76,7 +77,7 @@ define void @trip_count_8589935692() {
 ; CHECK-NEXT:  Loop %for.body: symbolic max backedge-taken count is 8589934591
 ; CHECK-NEXT:  Loop %for.body: Predicated backedge-taken count is 8589934591
 ; CHECK-NEXT:   Predicates:
-; CHECK:       Loop %for.body: Trip multiple is 1
+; CHECK:       Loop %for.body: Trip multiple is 2147483648
 ;
 entry:
   br label %for.body
@@ -105,7 +106,7 @@ define void @trip_count_9223372036854775808() {
 ; CHECK-NEXT:  Loop %for.body: symbolic max backedge-taken count is 9223372036854775807
 ; CHECK-NEXT:  Loop %for.body: Predicated backedge-taken count is 9223372036854775807
 ; CHECK-NEXT:   Predicates:
-; CHECK:       Loop %for.body: Trip multiple is 1
+; CHECK:       Loop %for.body: Trip multiple is 2147483648
 ;
 entry:
   br label %for.body

diff --git a/llvm/test/Transforms/LoopUnroll/X86/mmx.ll b/llvm/test/Transforms/LoopUnroll/X86/mmx.ll
index 9318bd1..979ce63 100644 (file)
--- a/llvm/test/Transforms/LoopUnroll/X86/mmx.ll
+++ b/llvm/test/Transforms/LoopUnroll/X86/mmx.ll
@@ -7,53 +7,21 @@ define x86_mmx @f() #0 {
 ; CHECK-LABEL: define x86_mmx @f
 ; CHECK-SAME: () #[[ATTR0:[0-9]+]] {
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    br i1 false, label [[EXIT_UNR_LCSSA:%.*]], label [[ENTRY_NEW:%.*]]
-; CHECK:       entry.new:
 ; CHECK-NEXT:    br label [[FOR_BODY:%.*]]
 ; CHECK:       for.body:
-; CHECK-NEXT:    [[PHI:%.*]] = phi i32 [ 1, [[ENTRY_NEW]] ], [ [[ADD_7:%.*]], [[FOR_BODY]] ]
-; CHECK-NEXT:    [[NITER:%.*]] = phi i32 [ 0, [[ENTRY_NEW]] ], [ [[NITER_NEXT_7:%.*]], [[FOR_BODY]] ]
+; CHECK-NEXT:    [[PHI:%.*]] = phi i32 [ 1, [[ENTRY:%.*]] ], [ [[ADD_7:%.*]], [[FOR_BODY]] ]
 ; CHECK-NEXT:    [[ADD:%.*]] = add nuw nsw i32 [[PHI]], 1
-; CHECK-NEXT:    [[NITER_NEXT:%.*]] = add nuw nsw i32 [[NITER]], 1
 ; CHECK-NEXT:    [[ADD_1:%.*]] = add nuw nsw i32 [[ADD]], 1
-; CHECK-NEXT:    [[NITER_NEXT_1:%.*]] = add nuw nsw i32 [[NITER_NEXT]], 1
 ; CHECK-NEXT:    [[ADD_2:%.*]] = add nuw nsw i32 [[ADD_1]], 1
-; CHECK-NEXT:    [[NITER_NEXT_2:%.*]] = add nuw nsw i32 [[NITER_NEXT_1]], 1
 ; CHECK-NEXT:    [[ADD_3:%.*]] = add nuw nsw i32 [[ADD_2]], 1
-; CHECK-NEXT:    [[NITER_NEXT_3:%.*]] = add nuw nsw i32 [[NITER_NEXT_2]], 1
 ; CHECK-NEXT:    [[ADD_4:%.*]] = add nuw nsw i32 [[ADD_3]], 1
-; CHECK-NEXT:    [[NITER_NEXT_4:%.*]] = add nuw nsw i32 [[NITER_NEXT_3]], 1
 ; CHECK-NEXT:    [[ADD_5:%.*]] = add nuw nsw i32 [[ADD_4]], 1
-; CHECK-NEXT:    [[NITER_NEXT_5:%.*]] = add nuw nsw i32 [[NITER_NEXT_4]], 1
 ; CHECK-NEXT:    [[ADD_6:%.*]] = add i32 [[ADD_5]], 1
-; CHECK-NEXT:    [[NITER_NEXT_6:%.*]] = add nuw nsw i32 [[NITER_NEXT_5]], 1
 ; CHECK-NEXT:    [[ADD_7]] = add nuw nsw i32 [[ADD_6]], 1
-; CHECK-NEXT:    [[NITER_NEXT_7]] = add i32 [[NITER_NEXT_6]], 1
-; CHECK-NEXT:    [[NITER_NCMP_7:%.*]] = icmp eq i32 [[NITER_NEXT_7]], 0
-; CHECK-NEXT:    br i1 [[NITER_NCMP_7]], label [[EXIT_UNR_LCSSA_LOOPEXIT:%.*]], label [[FOR_BODY]]
-; CHECK:       exit.unr-lcssa.loopexit:
-; CHECK-NEXT:    [[RET_PH_PH:%.*]] = phi x86_mmx [ undef, [[FOR_BODY]] ]
-; CHECK-NEXT:    [[PHI_UNR_PH:%.*]] = phi i32 [ [[ADD_7]], [[FOR_BODY]] ]
-; CHECK-NEXT:    br label [[EXIT_UNR_LCSSA]]
-; CHECK:       exit.unr-lcssa:
-; CHECK-NEXT:    [[RET_PH:%.*]] = phi x86_mmx [ undef, [[ENTRY:%.*]] ], [ [[RET_PH_PH]], [[EXIT_UNR_LCSSA_LOOPEXIT]] ]
-; CHECK-NEXT:    [[PHI_UNR:%.*]] = phi i32 [ 1, [[ENTRY]] ], [ [[PHI_UNR_PH]], [[EXIT_UNR_LCSSA_LOOPEXIT]] ]
-; CHECK-NEXT:    br i1 false, label [[FOR_BODY_EPIL_PREHEADER:%.*]], label [[EXIT:%.*]]
-; CHECK:       for.body.epil.preheader:
-; CHECK-NEXT:    br label [[FOR_BODY_EPIL:%.*]]
-; CHECK:       for.body.epil:
-; CHECK-NEXT:    [[PHI_EPIL:%.*]] = phi i32 [ [[PHI_UNR]], [[FOR_BODY_EPIL_PREHEADER]] ], [ [[ADD_EPIL:%.*]], [[FOR_BODY_EPIL]] ]
-; CHECK-NEXT:    [[EPIL_ITER:%.*]] = phi i32 [ 0, [[FOR_BODY_EPIL_PREHEADER]] ], [ [[EPIL_ITER_NEXT:%.*]], [[FOR_BODY_EPIL]] ]
-; CHECK-NEXT:    [[ADD_EPIL]] = add i32 [[PHI_EPIL]], 1
-; CHECK-NEXT:    [[CMP_EPIL:%.*]] = icmp eq i32 [[PHI_EPIL]], 0
-; CHECK-NEXT:    [[EPIL_ITER_NEXT]] = add i32 [[EPIL_ITER]], 1
-; CHECK-NEXT:    [[EPIL_ITER_CMP:%.*]] = icmp ne i32 [[EPIL_ITER_NEXT]], 0
-; CHECK-NEXT:    br i1 [[EPIL_ITER_CMP]], label [[FOR_BODY_EPIL]], label [[EXIT_EPILOG_LCSSA:%.*]], !llvm.loop [[LOOP0:![0-9]+]]
-; CHECK:       exit.epilog-lcssa:
-; CHECK-NEXT:    [[RET_PH1:%.*]] = phi x86_mmx [ undef, [[FOR_BODY_EPIL]] ]
-; CHECK-NEXT:    br label [[EXIT]]
+; CHECK-NEXT:    [[CMP_7:%.*]] = icmp eq i32 [[ADD_6]], 0
+; CHECK-NEXT:    br i1 [[CMP_7]], label [[EXIT:%.*]], label [[FOR_BODY]]
 ; CHECK:       exit:
-; CHECK-NEXT:    [[RET:%.*]] = phi x86_mmx [ [[RET_PH]], [[EXIT_UNR_LCSSA]] ], [ [[RET_PH1]], [[EXIT_EPILOG_LCSSA]] ]
+; CHECK-NEXT:    [[RET:%.*]] = phi x86_mmx [ undef, [[FOR_BODY]] ]
 ; CHECK-NEXT:    ret x86_mmx [[RET]]
 ;
 entry: