[ValueTracking] Extend r251146 to catch a fairly common case
Even though we may not know the value of the shifter operand, it's possible we know the shifter operand is non-zero. This can allow us to infer more known bits - for example:
%1 = load %p !range {1, 5}
%2 = shl %q, %1
We don't know %1, but we do know that it is nonzero so %2[0] is known zero, and importantly %2 is known non-zero.
Calling isKnownNonZero is nontrivially expensive so use an Optional to run it lazily and cache its result.
llvm-svn: 251294
diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index bb4220d..1171149 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -13,6 +13,7 @@
//===----------------------------------------------------------------------===//
#include "llvm/Analysis/ValueTracking.h"
+#include "llvm/ADT/Optional.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/Analysis/AssumptionCache.h"
#include "llvm/Analysis/InstructionSimplify.h"
@@ -1004,9 +1005,18 @@
// calculation. Reusing the APInts here to prevent unnecessary allocations.
KnownZero.clearAllBits(), KnownOne.clearAllBits();
+ // If we know the shifter operand is nonzero, we can sometimes infer more
+ // known bits. However this is expensive to compute, so be lazy about it and
+ // only compute it when absolutely necessary.
+ Optional<bool> ShifterOperandIsNonZero;
+
// Early exit if we can't constrain any well-defined shift amount.
- if (!(ShiftAmtKZ & (BitWidth-1)) && !(ShiftAmtKO & (BitWidth-1)))
- return;
+ if (!(ShiftAmtKZ & (BitWidth - 1)) && !(ShiftAmtKO & (BitWidth - 1))) {
+ ShifterOperandIsNonZero =
+ isKnownNonZero(I->getOperand(1), DL, Depth + 1, Q);
+ if (!*ShifterOperandIsNonZero)
+ return;
+ }
computeKnownBits(I->getOperand(0), KnownZero2, KnownOne2, DL, Depth + 1, Q);
@@ -1018,6 +1028,16 @@
continue;
if ((ShiftAmt | ShiftAmtKO) != ShiftAmt)
continue;
+ // If we know the shifter is nonzero, we may be able to infer more known
+ // bits. This check is sunk down as far as possible to avoid the expensive
+ // call to isKnownNonZero if the cheaper checks above fail.
+ if (ShiftAmt == 0) {
+ if (!ShifterOperandIsNonZero.hasValue())
+ ShifterOperandIsNonZero =
+ isKnownNonZero(I->getOperand(1), DL, Depth + 1, Q);
+ if (*ShifterOperandIsNonZero)
+ continue;
+ }
KnownZero &= KZF(KnownZero2, ShiftAmt);
KnownOne &= KOF(KnownOne2, ShiftAmt);