[SCEV] Use context instruction for SCEVUnknowns in getConstantMultiple. (llvm#163260)

Follow-up to llvm#160941.

Even if we don't have a context instruction for the caller, we should be
able to provide context instructions for SCEVUnknowns. Unless I am
missing something, SCEVUnknown only become available at the point their
underlying IR instruction has been defined. If it is an argument, it
should be safe to use the first instruction in the entry block or the
instruction itself if it wraps an instruction.

This allows getConstantMultiple to make better use of alignment
assumptions.

PR: llvm#163260
(cherry picked from commit 3b46556)

Author: fhahn

llvm/lib/Analysis/ScalarEvolution.cpp

@@ -6410,8 +6410,18 @@ APInt ScalarEvolution::getConstantMultipleImpl(const SCEV *S,
   case scSequentialUMinExpr:
     return GetGCDMultiple(cast<SCEVNAryExpr>(S));
   case scUnknown: {
-    // ask ValueTracking for known bits
+    // Ask ValueTracking for known bits. SCEVUnknown only become available at
+    // the point their underlying IR instruction has been defined. If CtxI was
+    // not provided, use:
+    // * the first instruction in the entry block if it is an argument
+    // * the instruction itself otherwise.
     const SCEVUnknown *U = cast<SCEVUnknown>(S);
+    if (!CtxI) {
+      if (isa<Argument>(U->getValue()))
+        CtxI = &*F.getEntryBlock().begin();
+      else if (auto *I = dyn_cast<Instruction>(U->getValue()))
+        CtxI = I;
+    }
     unsigned Known =
         computeKnownBits(U->getValue(), getDataLayout(), &AC, CtxI, &DT)
             .countMinTrailingZeros();

llvm/test/Analysis/ScalarEvolution/trip-multiple-guard-info.ll

@@ -578,22 +578,22 @@ define void @test_ptr_aligned_by_2_and_4_via_assumption(ptr %start, ptr %end) {
 ; CHECK-LABEL: 'test_ptr_aligned_by_2_and_4_via_assumption'
 ; CHECK-NEXT:  Classifying expressions for: @test_ptr_aligned_by_2_and_4_via_assumption
 ; CHECK-NEXT:    %iv = phi ptr [ %start, %entry ], [ %iv.next, %loop ]
-; CHECK-NEXT:    --> {%start,+,4}<%loop> U: full-set S: full-set Exits: <<Unknown>> LoopDispositions: { %loop: Computable }
+; CHECK-NEXT:    --> {%start,+,4}<%loop> U: [0,-1) S: [-9223372036854775808,9223372036854775807) Exits: <<Unknown>> LoopDispositions: { %loop: Computable }
 ; CHECK-NEXT:    %iv.next = getelementptr i8, ptr %iv, i64 4
-; CHECK-NEXT:    --> {(4 + %start),+,4}<%loop> U: full-set S: full-set Exits: <<Unknown>> LoopDispositions: { %loop: Computable }
+; CHECK-NEXT:    --> {(4 + %start),+,4}<%loop> U: [0,-1) S: [-9223372036854775808,9223372036854775807) Exits: <<Unknown>> LoopDispositions: { %loop: Computable }
 ; CHECK-NEXT:  Determining loop execution counts for: @test_ptr_aligned_by_2_and_4_via_assumption
 ; CHECK-NEXT:  Loop %loop: Unpredictable backedge-taken count.
 ; CHECK-NEXT:  Loop %loop: Unpredictable constant max backedge-taken count.
 ; CHECK-NEXT:  Loop %loop: Unpredictable symbolic max backedge-taken count.
 ; CHECK-NEXT:  Loop %loop: Predicated backedge-taken count is ((-4 + (-1 * (ptrtoint ptr %start to i64)) + (ptrtoint ptr %end to i64)) /u 4)
 ; CHECK-NEXT:   Predicates:
-; CHECK-NEXT:      Equal predicate: (zext i2 ((trunc i64 (ptrtoint ptr %end to i64) to i2) + (-1 * (trunc i64 (ptrtoint ptr %start to i64) to i2))) to i64) == 0
+; CHECK-NEXT:      Equal predicate: (zext i2 (-1 * (trunc i64 (ptrtoint ptr %start to i64) to i2)) to i64) == 0
 ; CHECK-NEXT:  Loop %loop: Predicated constant max backedge-taken count is i64 4611686018427387903
 ; CHECK-NEXT:   Predicates:
-; CHECK-NEXT:      Equal predicate: (zext i2 ((trunc i64 (ptrtoint ptr %end to i64) to i2) + (-1 * (trunc i64 (ptrtoint ptr %start to i64) to i2))) to i64) == 0
+; CHECK-NEXT:      Equal predicate: (zext i2 (-1 * (trunc i64 (ptrtoint ptr %start to i64) to i2)) to i64) == 0
 ; CHECK-NEXT:  Loop %loop: Predicated symbolic max backedge-taken count is ((-4 + (-1 * (ptrtoint ptr %start to i64)) + (ptrtoint ptr %end to i64)) /u 4)
 ; CHECK-NEXT:   Predicates:
-; CHECK-NEXT:      Equal predicate: (zext i2 ((trunc i64 (ptrtoint ptr %end to i64) to i2) + (-1 * (trunc i64 (ptrtoint ptr %start to i64) to i2))) to i64) == 0
+; CHECK-NEXT:      Equal predicate: (zext i2 (-1 * (trunc i64 (ptrtoint ptr %start to i64) to i2)) to i64) == 0
 ;
 entry:
   call void @llvm.assume(i1 true) [ "align"(ptr %start, i64 2) ]
@@ -615,9 +615,9 @@ define void @test_ptrs_aligned_by_4_via_assumption(ptr %start, ptr %end) {
 ; CHECK-LABEL: 'test_ptrs_aligned_by_4_via_assumption'
 ; CHECK-NEXT:  Classifying expressions for: @test_ptrs_aligned_by_4_via_assumption
 ; CHECK-NEXT:    %iv = phi ptr [ %start, %entry ], [ %iv.next, %loop ]
-; CHECK-NEXT:    --> {%start,+,4}<%loop> U: full-set S: full-set Exits: ((4 * ((-4 + (-1 * (ptrtoint ptr %start to i64)) + (ptrtoint ptr %end to i64)) /u 4))<nuw> + %start) LoopDispositions: { %loop: Computable }
+; CHECK-NEXT:    --> {%start,+,4}<%loop> U: [0,-3) S: [-9223372036854775808,9223372036854775805) Exits: (-4 + (-1 * (ptrtoint ptr %start to i64)) + (ptrtoint ptr %end to i64) + %start) LoopDispositions: { %loop: Computable }
 ; CHECK-NEXT:    %iv.next = getelementptr i8, ptr %iv, i64 4
-; CHECK-NEXT:    --> {(4 + %start),+,4}<%loop> U: full-set S: full-set Exits: (4 + (4 * ((-4 + (-1 * (ptrtoint ptr %start to i64)) + (ptrtoint ptr %end to i64)) /u 4))<nuw> + %start) LoopDispositions: { %loop: Computable }
+; CHECK-NEXT:    --> {(4 + %start),+,4}<%loop> U: [0,-3) S: [-9223372036854775808,9223372036854775805) Exits: ((-1 * (ptrtoint ptr %start to i64)) + (ptrtoint ptr %end to i64) + %start) LoopDispositions: { %loop: Computable }
 ; CHECK-NEXT:  Determining loop execution counts for: @test_ptrs_aligned_by_4_via_assumption
 ; CHECK-NEXT:  Loop %loop: backedge-taken count is ((-4 + (-1 * (ptrtoint ptr %start to i64)) + (ptrtoint ptr %end to i64)) /u 4)
 ; CHECK-NEXT:  Loop %loop: constant max backedge-taken count is i64 4611686018427387903
@@ -644,9 +644,9 @@ define void @test_ptrs_aligned_by_8_via_assumption(ptr %start, ptr %end) {
 ; CHECK-LABEL: 'test_ptrs_aligned_by_8_via_assumption'
 ; CHECK-NEXT:  Classifying expressions for: @test_ptrs_aligned_by_8_via_assumption
 ; CHECK-NEXT:    %iv = phi ptr [ %start, %entry ], [ %iv.next, %loop ]
-; CHECK-NEXT:    --> {%start,+,4}<%loop> U: full-set S: full-set Exits: ((4 * ((-4 + (-1 * (ptrtoint ptr %start to i64)) + (ptrtoint ptr %end to i64)) /u 4))<nuw> + %start) LoopDispositions: { %loop: Computable }
+; CHECK-NEXT:    --> {%start,+,4}<%loop> U: [0,-3) S: [-9223372036854775808,9223372036854775805) Exits: (-4 + (-1 * (ptrtoint ptr %start to i64)) + (ptrtoint ptr %end to i64) + %start) LoopDispositions: { %loop: Computable }
 ; CHECK-NEXT:    %iv.next = getelementptr i8, ptr %iv, i64 4
-; CHECK-NEXT:    --> {(4 + %start),+,4}<%loop> U: full-set S: full-set Exits: (4 + (4 * ((-4 + (-1 * (ptrtoint ptr %start to i64)) + (ptrtoint ptr %end to i64)) /u 4))<nuw> + %start) LoopDispositions: { %loop: Computable }
+; CHECK-NEXT:    --> {(4 + %start)<nuw><nsw>,+,4}<%loop> U: [0,-3) S: [-9223372036854775808,9223372036854775805) Exits: ((-1 * (ptrtoint ptr %start to i64)) + (ptrtoint ptr %end to i64) + %start) LoopDispositions: { %loop: Computable }
 ; CHECK-NEXT:  Determining loop execution counts for: @test_ptrs_aligned_by_8_via_assumption
 ; CHECK-NEXT:  Loop %loop: backedge-taken count is ((-4 + (-1 * (ptrtoint ptr %start to i64)) + (ptrtoint ptr %end to i64)) /u 4)
 ; CHECK-NEXT:  Loop %loop: constant max backedge-taken count is i64 4611686018427387903
@@ -677,22 +677,22 @@ define void @test_ptr_aligned_by_4_via_assumption_multiple_loop_predecessors(ptr
 ; CHECK-NEXT:    %c = call i1 @cond()
 ; CHECK-NEXT:    --> %c U: full-set S: full-set
 ; CHECK-NEXT:    %iv = phi ptr [ %start, %then ], [ %start, %else ], [ %iv.next, %loop ]
-; CHECK-NEXT:    --> {%start,+,4}<%loop> U: full-set S: full-set Exits: <<Unknown>> LoopDispositions: { %loop: Computable }
+; CHECK-NEXT:    --> {%start,+,4}<%loop> U: [0,-1) S: [-9223372036854775808,9223372036854775807) Exits: <<Unknown>> LoopDispositions: { %loop: Computable }
 ; CHECK-NEXT:    %iv.next = getelementptr i8, ptr %iv, i64 4
-; CHECK-NEXT:    --> {(4 + %start),+,4}<%loop> U: full-set S: full-set Exits: <<Unknown>> LoopDispositions: { %loop: Computable }
+; CHECK-NEXT:    --> {(4 + %start),+,4}<%loop> U: [0,-1) S: [-9223372036854775808,9223372036854775807) Exits: <<Unknown>> LoopDispositions: { %loop: Computable }
 ; CHECK-NEXT:  Determining loop execution counts for: @test_ptr_aligned_by_4_via_assumption_multiple_loop_predecessors
 ; CHECK-NEXT:  Loop %loop: Unpredictable backedge-taken count.
 ; CHECK-NEXT:  Loop %loop: Unpredictable constant max backedge-taken count.
 ; CHECK-NEXT:  Loop %loop: Unpredictable symbolic max backedge-taken count.
 ; CHECK-NEXT:  Loop %loop: Predicated backedge-taken count is ((-4 + (-1 * (ptrtoint ptr %start to i64)) + (ptrtoint ptr %end to i64)) /u 4)
 ; CHECK-NEXT:   Predicates:
-; CHECK-NEXT:      Equal predicate: (zext i2 ((trunc i64 (ptrtoint ptr %end to i64) to i2) + (-1 * (trunc i64 (ptrtoint ptr %start to i64) to i2))) to i64) == 0
+; CHECK-NEXT:      Equal predicate: (zext i2 (-1 * (trunc i64 (ptrtoint ptr %start to i64) to i2)) to i64) == 0
 ; CHECK-NEXT:  Loop %loop: Predicated constant max backedge-taken count is i64 4611686018427387903
 ; CHECK-NEXT:   Predicates:
-; CHECK-NEXT:      Equal predicate: (zext i2 ((trunc i64 (ptrtoint ptr %end to i64) to i2) + (-1 * (trunc i64 (ptrtoint ptr %start to i64) to i2))) to i64) == 0
+; CHECK-NEXT:      Equal predicate: (zext i2 (-1 * (trunc i64 (ptrtoint ptr %start to i64) to i2)) to i64) == 0
 ; CHECK-NEXT:  Loop %loop: Predicated symbolic max backedge-taken count is ((-4 + (-1 * (ptrtoint ptr %start to i64)) + (ptrtoint ptr %end to i64)) /u 4)
 ; CHECK-NEXT:   Predicates:
-; CHECK-NEXT:      Equal predicate: (zext i2 ((trunc i64 (ptrtoint ptr %end to i64) to i2) + (-1 * (trunc i64 (ptrtoint ptr %start to i64) to i2))) to i64) == 0
+; CHECK-NEXT:      Equal predicate: (zext i2 (-1 * (trunc i64 (ptrtoint ptr %start to i64) to i2)) to i64) == 0
 ;
 entry:
   call void @llvm.assume(i1 true) [ "align"(ptr %start, i64 2) ]