[llvm] 35b0955 - [ValueTracking] Added support to deduce PHI Nodes values being a power of 2

[William Huang via llvm-commits]

Author: William Huang
Date: 2022-05-26T20:30:31Z
New Revision: 35b0955aa59d3cf39678046ccff3728f6b7a91b0

URL: https://github.com/llvm/llvm-project/commit/35b0955aa59d3cf39678046ccff3728f6b7a91b0
DIFF: https://github.com/llvm/llvm-project/commit/35b0955aa59d3cf39678046ccff3728f6b7a91b0.diff

LOG: [ValueTracking] Added support to deduce PHI Nodes values being a power of 2

Add Value Tracking support to deduce induction variable being a power of 2, allowing urem optimizations

Reviewed By: davidxl

Differential Revision: https://reviews.llvm.org/D126018

Added: 
    

Modified: 
    llvm/lib/Analysis/ValueTracking.cpp
    llvm/test/Analysis/ValueTracking/known-power-of-two-urem.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index c7586f271df10..473b3cb74be83 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -2128,6 +2128,25 @@ bool isKnownToBeAPowerOfTwo(const Value *V, bool OrZero, unsigned Depth,
     }
   }
 
+  // A PHI node is power of two if all incoming values are power of two.
+  if (const PHINode *PN = dyn_cast<PHINode>(V)) {
+    Query RecQ = Q;
+
+    // Recursively check all incoming values. Limit recursion to 2 levels, so
+    // that search complexity is limited to number of operands^2.
+    unsigned NewDepth = std::max(Depth, MaxAnalysisRecursionDepth - 1);
+    return llvm::all_of(PN->operands(), [&](const Use &U) {
+      // Value is power of 2 if it is coming from PHI node itself by induction.
+      if (U.get() == PN)
+        return true;
+
+      // Change the context instruction to the incoming block where it is
+      // evaluated.
+      RecQ.CxtI = PN->getIncomingBlock(U)->getTerminator();
+      return isKnownToBeAPowerOfTwo(U.get(), OrZero, NewDepth, RecQ);
+    });
+  }
+
   // An exact divide or right shift can only shift off zero bits, so the result
   // is a power of two only if the first operand is a power of two and not
   // copying a sign bit (sdiv int_min, 2).

diff  --git a/llvm/test/Analysis/ValueTracking/known-power-of-two-urem.ll b/llvm/test/Analysis/ValueTracking/known-power-of-two-urem.ll
index 880e14dae8778..e25c769d6052b 100644
--- a/llvm/test/Analysis/ValueTracking/known-power-of-two-urem.ll
+++ b/llvm/test/Analysis/ValueTracking/known-power-of-two-urem.ll
@@ -19,7 +19,8 @@ define i64 @known_power_of_two_urem_phi(i64 %size, i1 %cmp, i1 %cmp1) {
 ; CHECK-NEXT:    br label [[COND_END]]
 ; CHECK:       cond.end:
 ; CHECK-NEXT:    [[PHI1:%.*]] = phi i64 [ 4096, [[ENTRY:%.*]] ], [ [[PHI]], [[COND_TRUE_END]] ]
-; CHECK-NEXT:    [[UREM:%.*]] = urem i64 [[SIZE:%.*]], [[PHI1]]
+; CHECK-NEXT:    [[TMP0:%.*]] = add nsw i64 [[PHI1]], -1
+; CHECK-NEXT:    [[UREM:%.*]] = and i64 [[TMP0]], [[SIZE:%.*]]
 ; CHECK-NEXT:    ret i64 [[UREM]]
 ;
 entry:
@@ -56,7 +57,8 @@ define i64 @known_power_of_two_urem_nested_expr(i64 %size, i1 %cmp, i1 %cmp1, i6
 ; CHECK-NEXT:    br label [[COND_END]]
 ; CHECK:       cond.end:
 ; CHECK-NEXT:    [[PHI:%.*]] = phi i64 [ [[SELECT]], [[COND_FALSE]] ], [ [[TMP1]], [[COND_TRUE]] ], [ [[PHI]], [[COND_END]] ]
-; CHECK-NEXT:    [[UREM:%.*]] = urem i64 [[SIZE:%.*]], [[PHI]]
+; CHECK-NEXT:    [[TMP2:%.*]] = add i64 [[PHI]], -1
+; CHECK-NEXT:    [[UREM:%.*]] = and i64 [[TMP2]], [[SIZE:%.*]]
 ; CHECK-NEXT:    [[CMP2:%.*]] = icmp ult i64 [[UREM]], 10
 ; CHECK-NEXT:    br i1 [[CMP2]], label [[COND_END]], label [[END:%.*]]
 ; CHECK:       end: