[llvm] [SCEV] Infer loop max trip count from memory accesses (PR #70361)

[Johannes Doerfert via llvm-commits]

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@@ -0,0 +1,110 @@
+; NOTE: Assertions have been autogenerated by utils/update_analyze_test_checks.py
+; RUN: opt < %s -disable-output "-passes=print<scalar-evolution>" -scalar-evolution-classify-expressions=0 -scalar-evolution-infer-max-trip-count-from-memory-access 2>&1 | FileCheck %s
+
+define void @ComputeMaxTripCountFromArrayIdxWrap(i32 signext %len) {
+; CHECK-LABEL: 'ComputeMaxTripCountFromArrayIdxWrap'
+; CHECK-NEXT:  Determining loop execution counts for: @ComputeMaxTripCountFromArrayIdxWrap
+; CHECK-NEXT:  Loop %for.body: backedge-taken count is (-1 + %len)
+; CHECK-NEXT:  Loop %for.body: constant max backedge-taken count is 2147483646
+; CHECK-NEXT:  Loop %for.body: symbolic max backedge-taken count is (-1 + %len)
+; CHECK-NEXT:  Loop %for.body: Predicated backedge-taken count is (-1 + %len)
+; CHECK-NEXT:   Predicates:
+; CHECK:       Loop %for.body: Trip multiple is 1
+; CHECK-NEXT:  Loop %for.body: Small constant max trip is 2147483647
+;
+entry:
+  %a = alloca [256 x i32], align 4
+  %cmp4 = icmp sgt i32 %len, 0
+  br i1 %cmp4, label %for.body.preheader, label %for.cond.cleanup
+
+for.body.preheader:
+  br label %for.body
+
+for.cond.cleanup.loopexit:
+  br label %for.cond.cleanup
+
+for.cond.cleanup:
+  ret void
+
+for.body:
+  %iv = phi i8 [ %inc, %for.body ], [ 0, %for.body.preheader ]
+  %idxprom = zext i8 %iv to i64
+  %arrayidx = getelementptr inbounds [256 x i32], [256 x i32]* %a, i64 0, i64 %idxprom
+  store i32 0, i32* %arrayidx, align 4
+  %inc = add nuw nsw i8 %iv, 1
+  %inc_zext = zext i8 %inc to i32
+  %cmp = icmp slt i32 %inc_zext, %len
+  br i1 %cmp, label %for.body, label %for.cond.cleanup.loopexit
+}
+
+define void @ComputeMaxTripCountFromArrayIdxWrap2(i32 signext %len) {
+; CHECK-LABEL: 'ComputeMaxTripCountFromArrayIdxWrap2'
+; CHECK-NEXT:  Determining loop execution counts for: @ComputeMaxTripCountFromArrayIdxWrap2
+; CHECK-NEXT:  Loop %for.body: backedge-taken count is (-1 + %len)
+; CHECK-NEXT:  Loop %for.body: constant max backedge-taken count is 2147483646
+; CHECK-NEXT:  Loop %for.body: symbolic max backedge-taken count is (-1 + %len)
+; CHECK-NEXT:  Loop %for.body: Predicated backedge-taken count is (-1 + %len)
+; CHECK-NEXT:   Predicates:
+; CHECK:       Loop %for.body: Trip multiple is 1
+; CHECK-NEXT:  Loop %for.body: Small constant max trip is 2147483647
+;
+entry:
+  %a = alloca [127 x i32], align 4
+  %cmp4 = icmp sgt i32 %len, 0
+  br i1 %cmp4, label %for.body.preheader, label %for.cond.cleanup
+
+for.body.preheader:
+  br label %for.body
+
+for.cond.cleanup.loopexit:
+  br label %for.cond.cleanup
+
+for.cond.cleanup:
+  ret void
+
+for.body:
+  %iv = phi i8 [ %inc, %for.body ], [ 0, %for.body.preheader ]
+  %idxprom = zext i8 %iv to i64
+  %arrayidx = getelementptr inbounds [127 x i32], [127 x i32]* %a, i64 0, i64 %idxprom
+  store i32 0, i32* %arrayidx, align 4
+  %inc = add nuw nsw i8 %iv, 1
+  %inc_zext = zext i8 %inc to i32
+  %cmp = icmp slt i32 %inc_zext, %len
+  br i1 %cmp, label %for.body, label %for.cond.cleanup.loopexit
+}
+
+define void @ComputeMaxTripCountFromArrayIdxWrap3(i32 signext %len) {
+; CHECK-LABEL: 'ComputeMaxTripCountFromArrayIdxWrap3'
+; CHECK-NEXT:  Determining loop execution counts for: @ComputeMaxTripCountFromArrayIdxWrap3
+; CHECK-NEXT:  Loop %for.body: backedge-taken count is (-1 + %len)
+; CHECK-NEXT:  Loop %for.body: constant max backedge-taken count is 2147483646
+; CHECK-NEXT:  Loop %for.body: symbolic max backedge-taken count is (-1 + %len)
+; CHECK-NEXT:  Loop %for.body: Predicated backedge-taken count is (-1 + %len)
+; CHECK-NEXT:   Predicates:
+; CHECK:       Loop %for.body: Trip multiple is 1
+; CHECK-NEXT:  Loop %for.body: Small constant max trip is 21
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jdoerfert wrote:

Shouldn't it be 20? Where is the off by one coming from?

https://github.com/llvm/llvm-project/pull/70361