[ValueTracking] Enhance DemandedElts support to PhiNode

[reames]

LGTM w/one required change.

llvm/test/Transforms/InstCombine/phi-known-bits-demandedelts.ll

1

Please use ./utils/update_test_checks.py

[xjtuhu]

update the test after using update_test_checks.py

[xjtuhu]

update the test after using update_test_checks.py

[xjtuhu]

llvm/test/Transforms/InstCombine/phi-known-bits-demandedelts.ll

1

Done!

[lebedev.ri]

This review seems to be stuck/dead, consider abandoning if no longer relevant.

llvm/lib/Analysis/ValueTracking.cpp

if (matchSimpleRecurrence(P, BO, R, L)) {
 
         unsigned OpNum = P->getOperand(0) == R ? 0 : 1;
         Instruction *RInst = P->getIncomingBlock(OpNum)->getTerminator();
         Instruction *LInst = P->getIncomingBlock(1-OpNum)->getTerminator();
 
         // Ok, we have a PHI of the form L op= R. Check for low
         // zero bits.
         RecQ.CxtI = RInst;
-        computeKnownBits(R, Known2, Depth + 1, RecQ);
+        computeKnownBits(R, DemandedElts, Known2, Depth + 1, RecQ);
 
         // We need to take the minimum number of known bits
         KnownBits Known3(BitWidth);
         RecQ.CxtI = LInst;
-        computeKnownBits(L, Known3, Depth + 1, RecQ);
+        computeKnownBits(L, DemandedElts, Known3, Depth + 1, RecQ);
 
         Known.Zero.setLowBits(std::min(Known2.countMinTrailingZeros(),
                                        Known3.countMinTrailingZeros()));
 
         auto *OverflowOp = dyn_cast<OverflowingBinaryOperator>(BO);
         if (OverflowOp && Q.IIQ.hasNoSignedWrap(OverflowOp)) {
           // If initial value of recurrence is nonnegative, and we are adding
           // a nonnegative number with nsw, the result can only be nonnegative

llvm/test/Transforms/InstCombine/phi-known-bits-demandedelts.ll

+; RUN: opt -instcombine -S < %s | FileCheck %s
+
+; CHECK: br i1 true, label %taken, label %nottaken
+declare void @foo4x32(<4 x i32>)
+
+define i1 @test(i32 %count) {
+entry:
+  br label %body
+
+body:
+  %index = phi i32 [ 0, %entry ], [ %index.next, %nottaken ]
+  %vec.ind = phi <4 x i32> [ <i32 0, i32 1, i32 2, i32 3>, %entry ], [ %vec.ind.next, %nottaken ]
+  call void @foo4x32(<4 x i32> %vec.ind)
+  %a = extractelement <4 x i32> %vec.ind, i32 0
+  %b = and i32 %a, 3
+  %cmp1 = icmp eq i32 %b, 0                    ;; should be always true
+  br i1 %cmp1, label %taken, label %nottaken
+
+taken:
+  ret i1 true
+
+nottaken:
+  %index.next = add i32 %index, 4
+  %vec.ind.next = add <4 x i32> %vec.ind, <i32 4, i32 4, i32 4, i32 4>
+  %cmp2 = icmp eq i32 %index.next, %count
+  br i1 %cmp2, label %end, label %body
+
+end:
+  ret i1 false
+}
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