[llvm] [clang-tools-extra] [LoopConstrainer] Apply loop gurads to check that loop bounds are safe (PR #71531)

Aleksandr Popov via cfe-commits cfe-commits at lists.llvm.org
Thu Jan 18 13:34:58 PST 2024


https://github.com/aleks-tmb updated https://github.com/llvm/llvm-project/pull/71531

>From c734c948599ace737d48b32a817bd6410bd2b13e Mon Sep 17 00:00:00 2001
From: Aleksander Popov <apopov at azul.com>
Date: Tue, 7 Nov 2023 05:31:13 +0100
Subject: [PATCH] [LoopConstrainer] Apply loop gurads to check that loop bounds
 are safe

Loop guards that apply to loop SCEV bounds allow IRCE for cases with
compound loop bounds such as:

if (K > 0 && M > 0)
  for (i = 0; i < min(K, M); i++) {...}

if (K > 0 && M > 0)
  for (i = min(K, M); i >= 0; i--) {...}

Otherwise SCEV couldn't prove that loops have safe bounds in these
cases.
---
 llvm/lib/Transforms/Utils/LoopConstrainer.cpp | 22 +++--
 .../Transforms/IRCE/compound-loop-bound.ll    | 85 +++++++++++++++++--
 2 files changed, 90 insertions(+), 17 deletions(-)

diff --git a/llvm/lib/Transforms/Utils/LoopConstrainer.cpp b/llvm/lib/Transforms/Utils/LoopConstrainer.cpp
index ea6d952cfa7d4f..da6f37df5b314a 100644
--- a/llvm/lib/Transforms/Utils/LoopConstrainer.cpp
+++ b/llvm/lib/Transforms/Utils/LoopConstrainer.cpp
@@ -42,8 +42,11 @@ static bool isSafeDecreasingBound(const SCEV *Start, const SCEV *BoundSCEV,
   ICmpInst::Predicate BoundPred =
       IsSigned ? CmpInst::ICMP_SGT : CmpInst::ICMP_UGT;
 
+  auto StartLG = SE.applyLoopGuards(Start, L);
+  auto BoundLG = SE.applyLoopGuards(BoundSCEV, L);
+
   if (LatchBrExitIdx == 1)
-    return SE.isLoopEntryGuardedByCond(L, BoundPred, Start, BoundSCEV);
+    return SE.isLoopEntryGuardedByCond(L, BoundPred, StartLG, BoundLG);
 
   assert(LatchBrExitIdx == 0 && "LatchBrExitIdx should be either 0 or 1");
 
@@ -54,10 +57,10 @@ static bool isSafeDecreasingBound(const SCEV *Start, const SCEV *BoundSCEV,
   const SCEV *Limit = SE.getMinusSCEV(SE.getConstant(Min), StepPlusOne);
 
   const SCEV *MinusOne =
-      SE.getMinusSCEV(BoundSCEV, SE.getOne(BoundSCEV->getType()));
+      SE.getMinusSCEV(BoundLG, SE.getOne(BoundLG->getType()));
 
-  return SE.isLoopEntryGuardedByCond(L, BoundPred, Start, MinusOne) &&
-         SE.isLoopEntryGuardedByCond(L, BoundPred, BoundSCEV, Limit);
+  return SE.isLoopEntryGuardedByCond(L, BoundPred, StartLG, MinusOne) &&
+         SE.isLoopEntryGuardedByCond(L, BoundPred, BoundLG, Limit);
 }
 
 /// Given a loop with an increasing induction variable, is it possible to
@@ -86,8 +89,11 @@ static bool isSafeIncreasingBound(const SCEV *Start, const SCEV *BoundSCEV,
   ICmpInst::Predicate BoundPred =
       IsSigned ? CmpInst::ICMP_SLT : CmpInst::ICMP_ULT;
 
+  auto StartLG = SE.applyLoopGuards(Start, L);
+  auto BoundLG = SE.applyLoopGuards(BoundSCEV, L);
+
   if (LatchBrExitIdx == 1)
-    return SE.isLoopEntryGuardedByCond(L, BoundPred, Start, BoundSCEV);
+    return SE.isLoopEntryGuardedByCond(L, BoundPred, StartLG, BoundLG);
 
   assert(LatchBrExitIdx == 0 && "LatchBrExitIdx should be 0 or 1");
 
@@ -97,9 +103,9 @@ static bool isSafeIncreasingBound(const SCEV *Start, const SCEV *BoundSCEV,
                        : APInt::getMaxValue(BitWidth);
   const SCEV *Limit = SE.getMinusSCEV(SE.getConstant(Max), StepMinusOne);
 
-  return (SE.isLoopEntryGuardedByCond(L, BoundPred, Start,
-                                      SE.getAddExpr(BoundSCEV, Step)) &&
-          SE.isLoopEntryGuardedByCond(L, BoundPred, BoundSCEV, Limit));
+  return (SE.isLoopEntryGuardedByCond(L, BoundPred, StartLG,
+                                      SE.getAddExpr(BoundLG, Step)) &&
+          SE.isLoopEntryGuardedByCond(L, BoundPred, BoundLG, Limit));
 }
 
 /// Returns estimate for max latch taken count of the loop of the narrowest
diff --git a/llvm/test/Transforms/IRCE/compound-loop-bound.ll b/llvm/test/Transforms/IRCE/compound-loop-bound.ll
index 0930d19e22154f..e50d8c6127f401 100644
--- a/llvm/test/Transforms/IRCE/compound-loop-bound.ll
+++ b/llvm/test/Transforms/IRCE/compound-loop-bound.ll
@@ -16,23 +16,56 @@ define void @incrementing_loop(ptr %arr, ptr %len_ptr, i32 %K, i32 %M) {
 ; CHECK-NEXT:    br i1 [[AND]], label [[PREHEADER:%.*]], label [[EXIT:%.*]]
 ; CHECK:       preheader:
 ; CHECK-NEXT:    [[SMIN:%.*]] = call i32 @llvm.smin.i32(i32 [[K]], i32 [[M]])
+; CHECK-NEXT:    [[SMIN1:%.*]] = call i32 @llvm.smin.i32(i32 [[LEN]], i32 [[M]])
+; CHECK-NEXT:    [[SMIN2:%.*]] = call i32 @llvm.smin.i32(i32 [[SMIN1]], i32 [[K]])
+; CHECK-NEXT:    [[EXIT_MAINLOOP_AT:%.*]] = call i32 @llvm.smax.i32(i32 [[SMIN2]], i32 0)
+; CHECK-NEXT:    [[TMP0:%.*]] = icmp slt i32 0, [[EXIT_MAINLOOP_AT]]
+; CHECK-NEXT:    br i1 [[TMP0]], label [[LOOP_PREHEADER:%.*]], label [[MAIN_PSEUDO_EXIT:%.*]]
+; CHECK:       loop.preheader:
 ; CHECK-NEXT:    br label [[LOOP:%.*]]
 ; CHECK:       loop:
-; CHECK-NEXT:    [[IDX:%.*]] = phi i32 [ 0, [[PREHEADER]] ], [ [[IDX_NEXT:%.*]], [[IN_BOUNDS:%.*]] ]
-; CHECK-NEXT:    [[IDX_NEXT]] = add i32 [[IDX]], 1
+; CHECK-NEXT:    [[IDX:%.*]] = phi i32 [ [[IDX_NEXT:%.*]], [[IN_BOUNDS:%.*]] ], [ 0, [[LOOP_PREHEADER]] ]
+; CHECK-NEXT:    [[IDX_NEXT]] = add nsw i32 [[IDX]], 1
 ; CHECK-NEXT:    [[GUARD:%.*]] = icmp slt i32 [[IDX]], [[LEN]]
-; CHECK-NEXT:    br i1 [[GUARD]], label [[IN_BOUNDS]], label [[OUT_OF_BOUNDS:%.*]]
+; CHECK-NEXT:    br i1 true, label [[IN_BOUNDS]], label [[OUT_OF_BOUNDS_LOOPEXIT3:%.*]]
 ; CHECK:       in.bounds:
 ; CHECK-NEXT:    [[ADDR:%.*]] = getelementptr i32, ptr [[ARR]], i32 [[IDX]]
 ; CHECK-NEXT:    store i32 0, ptr [[ADDR]], align 4
 ; CHECK-NEXT:    [[NEXT:%.*]] = icmp slt i32 [[IDX_NEXT]], [[SMIN]]
-; CHECK-NEXT:    br i1 [[NEXT]], label [[LOOP]], label [[EXIT_LOOPEXIT:%.*]]
+; CHECK-NEXT:    [[TMP1:%.*]] = icmp slt i32 [[IDX_NEXT]], [[EXIT_MAINLOOP_AT]]
+; CHECK-NEXT:    br i1 [[TMP1]], label [[LOOP]], label [[MAIN_EXIT_SELECTOR:%.*]]
+; CHECK:       main.exit.selector:
+; CHECK-NEXT:    [[IDX_NEXT_LCSSA:%.*]] = phi i32 [ [[IDX_NEXT]], [[IN_BOUNDS]] ]
+; CHECK-NEXT:    [[TMP2:%.*]] = icmp slt i32 [[IDX_NEXT_LCSSA]], [[SMIN]]
+; CHECK-NEXT:    br i1 [[TMP2]], label [[MAIN_PSEUDO_EXIT]], label [[EXIT_LOOPEXIT:%.*]]
+; CHECK:       main.pseudo.exit:
+; CHECK-NEXT:    [[IDX_COPY:%.*]] = phi i32 [ 0, [[PREHEADER]] ], [ [[IDX_NEXT_LCSSA]], [[MAIN_EXIT_SELECTOR]] ]
+; CHECK-NEXT:    [[INDVAR_END:%.*]] = phi i32 [ 0, [[PREHEADER]] ], [ [[IDX_NEXT_LCSSA]], [[MAIN_EXIT_SELECTOR]] ]
+; CHECK-NEXT:    br label [[POSTLOOP:%.*]]
+; CHECK:       out.of.bounds.loopexit:
+; CHECK-NEXT:    br label [[OUT_OF_BOUNDS:%.*]]
+; CHECK:       out.of.bounds.loopexit3:
+; CHECK-NEXT:    br label [[OUT_OF_BOUNDS]]
 ; CHECK:       out.of.bounds:
 ; CHECK-NEXT:    ret void
+; CHECK:       exit.loopexit.loopexit:
+; CHECK-NEXT:    br label [[EXIT_LOOPEXIT]]
 ; CHECK:       exit.loopexit:
 ; CHECK-NEXT:    br label [[EXIT]]
 ; CHECK:       exit:
 ; CHECK-NEXT:    ret void
+; CHECK:       postloop:
+; CHECK-NEXT:    br label [[LOOP_POSTLOOP:%.*]]
+; CHECK:       loop.postloop:
+; CHECK-NEXT:    [[IDX_POSTLOOP:%.*]] = phi i32 [ [[IDX_COPY]], [[POSTLOOP]] ], [ [[IDX_NEXT_POSTLOOP:%.*]], [[IN_BOUNDS_POSTLOOP:%.*]] ]
+; CHECK-NEXT:    [[IDX_NEXT_POSTLOOP]] = add i32 [[IDX_POSTLOOP]], 1
+; CHECK-NEXT:    [[GUARD_POSTLOOP:%.*]] = icmp slt i32 [[IDX_POSTLOOP]], [[LEN]]
+; CHECK-NEXT:    br i1 [[GUARD_POSTLOOP]], label [[IN_BOUNDS_POSTLOOP]], label [[OUT_OF_BOUNDS_LOOPEXIT:%.*]]
+; CHECK:       in.bounds.postloop:
+; CHECK-NEXT:    [[ADDR_POSTLOOP:%.*]] = getelementptr i32, ptr [[ARR]], i32 [[IDX_POSTLOOP]]
+; CHECK-NEXT:    store i32 0, ptr [[ADDR_POSTLOOP]], align 4
+; CHECK-NEXT:    [[NEXT_POSTLOOP:%.*]] = icmp slt i32 [[IDX_NEXT_POSTLOOP]], [[SMIN]]
+; CHECK-NEXT:    br i1 [[NEXT_POSTLOOP]], label [[LOOP_POSTLOOP]], label [[EXIT_LOOPEXIT_LOOPEXIT:%.*]], !llvm.loop [[LOOP1:![0-9]+]], !loop_constrainer.loop.clone !6
 ;
 entry:
   %len = load i32, ptr %len_ptr, !range !0
@@ -78,24 +111,58 @@ define void @decrementing_loop(ptr %arr, ptr %len_ptr, i32 %K, i32 %M) {
 ; CHECK-NEXT:    [[AND:%.*]] = and i1 [[CHECK0]], [[CHECK1]]
 ; CHECK-NEXT:    br i1 [[AND]], label [[PREHEADER:%.*]], label [[EXIT:%.*]]
 ; CHECK:       preheader:
-; CHECK-NEXT:    [[SMIN:%.*]] = call i32 @llvm.smin.i32(i32 [[K]], i32 [[M]])
+; CHECK-NEXT:    [[INDVAR_START:%.*]] = call i32 @llvm.smin.i32(i32 [[K]], i32 [[M]])
+; CHECK-NEXT:    [[TMP0:%.*]] = add i32 [[INDVAR_START]], 1
+; CHECK-NEXT:    [[SMIN:%.*]] = call i32 @llvm.smin.i32(i32 [[LEN]], i32 [[TMP0]])
+; CHECK-NEXT:    [[SMAX:%.*]] = call i32 @llvm.smax.i32(i32 [[SMIN]], i32 0)
+; CHECK-NEXT:    [[EXIT_PRELOOP_AT:%.*]] = add nsw i32 [[SMAX]], -1
+; CHECK-NEXT:    [[TMP1:%.*]] = icmp sgt i32 [[INDVAR_START]], [[EXIT_PRELOOP_AT]]
+; CHECK-NEXT:    br i1 [[TMP1]], label [[LOOP_PRELOOP_PREHEADER:%.*]], label [[PRELOOP_PSEUDO_EXIT:%.*]]
+; CHECK:       loop.preloop.preheader:
+; CHECK-NEXT:    br label [[LOOP_PRELOOP:%.*]]
+; CHECK:       mainloop:
 ; CHECK-NEXT:    br label [[LOOP:%.*]]
 ; CHECK:       loop:
-; CHECK-NEXT:    [[IDX:%.*]] = phi i32 [ [[SMIN]], [[PREHEADER]] ], [ [[IDX_DEC:%.*]], [[IN_BOUNDS:%.*]] ]
-; CHECK-NEXT:    [[IDX_DEC]] = sub i32 [[IDX]], 1
+; CHECK-NEXT:    [[IDX:%.*]] = phi i32 [ [[IDX_PRELOOP_COPY:%.*]], [[MAINLOOP:%.*]] ], [ [[IDX_DEC:%.*]], [[IN_BOUNDS:%.*]] ]
+; CHECK-NEXT:    [[IDX_DEC]] = sub nsw i32 [[IDX]], 1
 ; CHECK-NEXT:    [[GUARD:%.*]] = icmp slt i32 [[IDX]], [[LEN]]
-; CHECK-NEXT:    br i1 [[GUARD]], label [[IN_BOUNDS]], label [[OUT_OF_BOUNDS:%.*]]
+; CHECK-NEXT:    br i1 true, label [[IN_BOUNDS]], label [[OUT_OF_BOUNDS_LOOPEXIT1:%.*]]
 ; CHECK:       in.bounds:
 ; CHECK-NEXT:    [[ADDR:%.*]] = getelementptr i32, ptr [[ARR]], i32 [[IDX]]
 ; CHECK-NEXT:    store i32 0, ptr [[ADDR]], align 4
 ; CHECK-NEXT:    [[NEXT:%.*]] = icmp sgt i32 [[IDX_DEC]], -1
-; CHECK-NEXT:    br i1 [[NEXT]], label [[LOOP]], label [[EXIT_LOOPEXIT:%.*]]
+; CHECK-NEXT:    br i1 [[NEXT]], label [[LOOP]], label [[EXIT_LOOPEXIT_LOOPEXIT:%.*]]
+; CHECK:       out.of.bounds.loopexit:
+; CHECK-NEXT:    br label [[OUT_OF_BOUNDS:%.*]]
+; CHECK:       out.of.bounds.loopexit1:
+; CHECK-NEXT:    br label [[OUT_OF_BOUNDS]]
 ; CHECK:       out.of.bounds:
 ; CHECK-NEXT:    ret void
+; CHECK:       exit.loopexit.loopexit:
+; CHECK-NEXT:    br label [[EXIT_LOOPEXIT:%.*]]
 ; CHECK:       exit.loopexit:
 ; CHECK-NEXT:    br label [[EXIT]]
 ; CHECK:       exit:
 ; CHECK-NEXT:    ret void
+; CHECK:       loop.preloop:
+; CHECK-NEXT:    [[IDX_PRELOOP:%.*]] = phi i32 [ [[IDX_DEC_PRELOOP:%.*]], [[IN_BOUNDS_PRELOOP:%.*]] ], [ [[INDVAR_START]], [[LOOP_PRELOOP_PREHEADER]] ]
+; CHECK-NEXT:    [[IDX_DEC_PRELOOP]] = sub i32 [[IDX_PRELOOP]], 1
+; CHECK-NEXT:    [[GUARD_PRELOOP:%.*]] = icmp slt i32 [[IDX_PRELOOP]], [[LEN]]
+; CHECK-NEXT:    br i1 [[GUARD_PRELOOP]], label [[IN_BOUNDS_PRELOOP]], label [[OUT_OF_BOUNDS_LOOPEXIT:%.*]]
+; CHECK:       in.bounds.preloop:
+; CHECK-NEXT:    [[ADDR_PRELOOP:%.*]] = getelementptr i32, ptr [[ARR]], i32 [[IDX_PRELOOP]]
+; CHECK-NEXT:    store i32 0, ptr [[ADDR_PRELOOP]], align 4
+; CHECK-NEXT:    [[NEXT_PRELOOP:%.*]] = icmp sgt i32 [[IDX_DEC_PRELOOP]], -1
+; CHECK-NEXT:    [[TMP2:%.*]] = icmp sgt i32 [[IDX_DEC_PRELOOP]], [[EXIT_PRELOOP_AT]]
+; CHECK-NEXT:    br i1 [[TMP2]], label [[LOOP_PRELOOP]], label [[PRELOOP_EXIT_SELECTOR:%.*]], !llvm.loop [[LOOP7:![0-9]+]], !loop_constrainer.loop.clone !6
+; CHECK:       preloop.exit.selector:
+; CHECK-NEXT:    [[IDX_DEC_PRELOOP_LCSSA:%.*]] = phi i32 [ [[IDX_DEC_PRELOOP]], [[IN_BOUNDS_PRELOOP]] ]
+; CHECK-NEXT:    [[TMP3:%.*]] = icmp sgt i32 [[IDX_DEC_PRELOOP_LCSSA]], -1
+; CHECK-NEXT:    br i1 [[TMP3]], label [[PRELOOP_PSEUDO_EXIT]], label [[EXIT_LOOPEXIT]]
+; CHECK:       preloop.pseudo.exit:
+; CHECK-NEXT:    [[IDX_PRELOOP_COPY]] = phi i32 [ [[INDVAR_START]], [[PREHEADER]] ], [ [[IDX_DEC_PRELOOP_LCSSA]], [[PRELOOP_EXIT_SELECTOR]] ]
+; CHECK-NEXT:    [[INDVAR_END:%.*]] = phi i32 [ [[INDVAR_START]], [[PREHEADER]] ], [ [[IDX_DEC_PRELOOP_LCSSA]], [[PRELOOP_EXIT_SELECTOR]] ]
+; CHECK-NEXT:    br label [[MAINLOOP]]
 ;
   entry:
   %len = load i32, ptr %len_ptr, !range !0



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