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[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
This commit is contained in:
William Huang 2022-05-26 20:15:00 +00:00
parent c4c750058f
commit 35b0955aa5
2 changed files with 23 additions and 2 deletions
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19
llvm/lib/Analysis/ValueTracking.cpp
View File

@ -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).

6
llvm/test/Analysis/ValueTracking/known-power-of-two-urem.ll
View File

@ -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: