[llvm] a836de0 - [SCEV] Compute ranges for ashr recurrences

[Philip Reames via llvm-commits]

Author: Philip Reames
Date: 2021-04-28T12:36:20-07:00
New Revision: a836de0bdef2ed25e46bd304f3a53a1f08be51c4

URL: https://github.com/llvm/llvm-project/commit/a836de0bdef2ed25e46bd304f3a53a1f08be51c4
DIFF: https://github.com/llvm/llvm-project/commit/a836de0bdef2ed25e46bd304f3a53a1f08be51c4.diff

LOG: [SCEV] Compute ranges for ashr recurrences

Straight forward extension to the recently added infrastructure which was pioneered with shl. This was originally posted as part of D99687, but split off for ease of review.

(I also decided to exclude the unknown start sign case explicitly for simplicity of understanding.)

Differential Revision: https://reviews.llvm.org/D101181

Added: 
    

Modified: 
    llvm/lib/Analysis/ScalarEvolution.cpp
    llvm/test/Analysis/ScalarEvolution/shift-recurrences.ll

Removed: 
    


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diff  --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index 4de5abfdc3f17..c2bf309e05913 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -5692,10 +5692,11 @@ getRangeForUnknownRecurrence(const SCEVUnknown *U) {
     // until the caller issue can be fixed.  PR49566 tracks the bug.
     return CR;
 
-  // TODO: Extend to other opcodes such as ashr, mul, and div
+  // TODO: Extend to other opcodes such as mul, and div
   switch (BO->getOpcode()) {
   default:
     return CR;
+  case Instruction::AShr:
   case Instruction::LShr:
   case Instruction::Shl:
     break;
@@ -5725,6 +5726,27 @@ getRangeForUnknownRecurrence(const SCEVUnknown *U) {
   switch (BO->getOpcode()) {
   default:
     llvm_unreachable("filtered out above");
+  case Instruction::AShr: {
+    // For each ashr, three cases:
+    //   shift = 0 => unchanged value
+    //   saturation => 0 or -1
+    //   other => a value closer to zero (of the same sign)
+    // Thus, the end value is closer to zero than the start.
+    auto KnownEnd = KnownBits::ashr(KnownStart,
+                                    KnownBits::makeConstant(TotalShift));
+    if (KnownStart.isNonNegative()) {
+      // Analogous to lshr (simply not yet canonicalized)
+      auto R = ConstantRange::getNonEmpty(KnownEnd.getMinValue(),
+                                          KnownStart.getMaxValue() + 1);
+      CR = CR.intersectWith(R);
+    } else if (KnownStart.isNegative()) {
+      // End >=u Start && End <=s Start
+      auto R = ConstantRange::getNonEmpty(KnownStart.getMinValue(),
+                                          KnownEnd.getMaxValue() + 1);
+      CR = CR.intersectWith(R);
+    }
+    break;
+  }
   case Instruction::LShr: {
     // For each lshr, three cases:
     //   shift = 0 => unchanged value

diff  --git a/llvm/test/Analysis/ScalarEvolution/shift-recurrences.ll b/llvm/test/Analysis/ScalarEvolution/shift-recurrences.ll
index 2ed80c7f0a430..568e014605cfe 100644
--- a/llvm/test/Analysis/ScalarEvolution/shift-recurrences.ll
+++ b/llvm/test/Analysis/ScalarEvolution/shift-recurrences.ll
@@ -503,7 +503,7 @@ define void @test_ashr_tc_positive() {
 ; CHECK-NEXT:    %iv = phi i64 [ 0, %entry ], [ %iv.next, %loop ]
 ; CHECK-NEXT:    --> {0,+,1}<%loop> U: [0,5) S: [0,5) Exits: 4 LoopDispositions: { %loop: Computable }
 ; CHECK-NEXT:    %iv.ashr = phi i64 [ 1023, %entry ], [ %iv.ashr.next, %loop ]
-; CHECK-NEXT:    --> %iv.ashr U: [0,1024) S: [0,1024) Exits: 63 LoopDispositions: { %loop: Variant }
+; CHECK-NEXT:    --> %iv.ashr U: [63,1024) S: [63,1024) Exits: 63 LoopDispositions: { %loop: Variant }
 ; CHECK-NEXT:    %iv.next = add i64 %iv, 1
 ; CHECK-NEXT:    --> {1,+,1}<%loop> U: [1,6) S: [1,6) Exits: 5 LoopDispositions: { %loop: Computable }
 ; CHECK-NEXT:    %iv.ashr.next = ashr i64 %iv.ashr, 1
@@ -534,7 +534,7 @@ define void @test_ashr_tc_negative() {
 ; CHECK-NEXT:    %iv = phi i64 [ 0, %entry ], [ %iv.next, %loop ]
 ; CHECK-NEXT:    --> {0,+,1}<%loop> U: [0,5) S: [0,5) Exits: 4 LoopDispositions: { %loop: Computable }
 ; CHECK-NEXT:    %iv.ashr = phi i8 [ -128, %entry ], [ %iv.ashr.next, %loop ]
-; CHECK-NEXT:    --> %iv.ashr U: [-128,0) S: [-128,0) Exits: -8 LoopDispositions: { %loop: Variant }
+; CHECK-NEXT:    --> %iv.ashr U: [-128,-7) S: [-128,-7) Exits: -8 LoopDispositions: { %loop: Variant }
 ; CHECK-NEXT:    %iv.next = add i64 %iv, 1
 ; CHECK-NEXT:    --> {1,+,1}<%loop> U: [1,6) S: [1,6) Exits: 5 LoopDispositions: { %loop: Computable }
 ; CHECK-NEXT:    %iv.ashr.next = ashr i8 %iv.ashr, 1
@@ -599,11 +599,11 @@ define void @test_ashr_zero_shift() {
 ; CHECK-NEXT:    %iv = phi i64 [ 0, %entry ], [ %iv.next, %loop ]
 ; CHECK-NEXT:    --> {0,+,1}<%loop> U: [0,5) S: [0,5) Exits: 4 LoopDispositions: { %loop: Computable }
 ; CHECK-NEXT:    %iv.ashr = phi i64 [ 1023, %entry ], [ %iv.ashr.next, %loop ]
-; CHECK-NEXT:    --> %iv.ashr U: [0,1024) S: [0,1024) Exits: 1023 LoopDispositions: { %loop: Variant }
+; CHECK-NEXT:    --> %iv.ashr U: [1023,1024) S: [1023,1024) Exits: 1023 LoopDispositions: { %loop: Variant }
 ; CHECK-NEXT:    %iv.next = add i64 %iv, 1
 ; CHECK-NEXT:    --> {1,+,1}<%loop> U: [1,6) S: [1,6) Exits: 5 LoopDispositions: { %loop: Computable }
 ; CHECK-NEXT:    %iv.ashr.next = ashr i64 %iv.ashr, 0
-; CHECK-NEXT:    --> %iv.ashr U: [0,1024) S: [0,1024) Exits: 1023 LoopDispositions: { %loop: Variant }
+; CHECK-NEXT:    --> %iv.ashr U: [1023,1024) S: [1023,1024) Exits: 1023 LoopDispositions: { %loop: Variant }
 ; CHECK-NEXT:  Determining loop execution counts for: @test_ashr_zero_shift
 ; CHECK-NEXT:  Loop %loop: backedge-taken count is 4
 ; CHECK-NEXT:  Loop %loop: max backedge-taken count is 4