[AggressiveInstcombine] Fold away shift in or reduction chain.

[davemgreen]

If we have icmp eq or(a, shl(b)), 0 then the shift can be removed so long as it is nuw or nsw. It is still comparing that some bits are non-zero.
https://alive2.llvm.org/ce/z/nhrBVX.

This is also true of ne, and true for longer or chains.

[llvmbot]

@llvm/pr-subscribers-llvm-transforms

Author: David Green (davemgreen)

Changes

If we have icmp eq or(a, shl(b)), 0 then the shift can be removed so long as it is nuw or nsw. It is still comparing that some bits are non-zero.
https://alive2.llvm.org/ce/z/nhrBVX.

This is also true of ne, and true for longer or chains.

Thinking out loud, this is kind of like a "any-bit is demanded" combine. I'm not sure if that exists already.

---

Full diff: https://github.com/llvm/llvm-project/pull/137875.diff

2 Files Affected:

• (modified) llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp (+28)
• (modified) llvm/test/Transforms/InstCombine/icmp-of-or-x.ll (+109)

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
index b7b0bb7361359..d2aa957b10b78 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -5046,6 +5046,29 @@ static Instruction *foldICmpOrXX(ICmpInst &I, const SimplifyQuery &Q,
   return nullptr;
 }
 
+static Value *foldShiftAwayFromOrChain(Instruction &I,
+                                       InstCombiner::BuilderTy &Builder) {
+  if (I.getOpcode() != Instruction::Or)
+    return nullptr;
+  Value *A, *B;
+  if (match(&I, m_c_Or(m_CombineOr(m_NSWShl(m_Value(A), m_Value()),
+                                   m_NUWShl(m_Value(A), m_Value())),
+                       m_Value(B))))
+    return Builder.CreateOr(A, B);
+
+  Value *Op0 = I.getOperand(0);
+  if (isa<Instruction>(Op0))
+    if (auto *X = foldShiftAwayFromOrChain(*cast<Instruction>(Op0), Builder))
+      Op0 = X;
+  Value *Op1 = I.getOperand(1);
+  if (isa<Instruction>(Op1))
+    if (auto *X = foldShiftAwayFromOrChain(*cast<Instruction>(Op1), Builder))
+      Op1 = X;
+  if (Op0 != I.getOperand(0) || Op1 != I.getOperand(1))
+    return Builder.CreateOr(Op0, Op1);
+  return nullptr;
+}
+
 static Instruction *foldICmpXorXX(ICmpInst &I, const SimplifyQuery &Q,
                                   InstCombinerImpl &IC) {
   Value *Op0 = I.getOperand(0), *Op1 = I.getOperand(1), *A;
@@ -7742,6 +7765,11 @@ Instruction *InstCombinerImpl::visitICmpInst(ICmpInst &I) {
     }
   }
 
+  // icmp eq/ne or(shl(a), b), 0 -> icmp eq/ne or(a, b)
+  if (I.isEquality() && match(Op1, m_Zero()) && isa<Instruction>(Op0))
+    if (auto *Res = foldShiftAwayFromOrChain(*cast<Instruction>(Op0), Builder))
+      return new ICmpInst(I.getPredicate(), Res, Op1);
+
   return Changed ? &I : nullptr;
 }
 
diff --git a/llvm/test/Transforms/InstCombine/icmp-of-or-x.ll b/llvm/test/Transforms/InstCombine/icmp-of-or-x.ll
index 993325f6ff0b0..baaa754e52894 100644
--- a/llvm/test/Transforms/InstCombine/icmp-of-or-x.ll
+++ b/llvm/test/Transforms/InstCombine/icmp-of-or-x.ll
@@ -409,3 +409,112 @@ define i1 @PR38139(i8 %arg) {
   %r = icmp ne i8 %masked, %arg
   ret i1 %r
 }
+
+define i1 @remove_shift_nuw_ab(i8 %a, i8 %b, i8 %s) {
+; CHECK-LABEL: @remove_shift_nuw_ab(
+; CHECK-NEXT:    [[OR:%.*]] = or i8 [[T:%.*]], [[B:%.*]]
+; CHECK-NEXT:    [[IC:%.*]] = icmp eq i8 [[OR]], 0
+; CHECK-NEXT:    ret i1 [[IC]]
+;
+  %t = shl nuw i8 %a, %s
+  %or = or i8 %t, %b
+  %ic = icmp eq i8 %or, 0
+  ret i1 %ic
+}
+
+define i1 @remove_shift_nuw_ba(i8 %a, i8 %b, i8 %s) {
+; CHECK-LABEL: @remove_shift_nuw_ba(
+; CHECK-NEXT:    [[OR:%.*]] = or i8 [[B:%.*]], [[T:%.*]]
+; CHECK-NEXT:    [[IC:%.*]] = icmp eq i8 [[OR]], 0
+; CHECK-NEXT:    ret i1 [[IC]]
+;
+  %t = shl nuw i8 %a, %s
+  %or = or i8 %b, %t
+  %ic = icmp eq i8 %or, 0
+  ret i1 %ic
+}
+
+define i1 @remove_shift_nsw(i8 %a, i8 %b, i8 %s) {
+; CHECK-LABEL: @remove_shift_nsw(
+; CHECK-NEXT:    [[OR:%.*]] = or i8 [[T:%.*]], [[B:%.*]]
+; CHECK-NEXT:    [[IC:%.*]] = icmp eq i8 [[OR]], 0
+; CHECK-NEXT:    ret i1 [[IC]]
+;
+  %t = shl nsw i8 %a, %s
+  %or = or i8 %t, %b
+  %ic = icmp eq i8 %or, 0
+  ret i1 %ic
+}
+
+define i1 @remove_shift_nuw_ne(i8 %a, i8 %b, i8 %s) {
+; CHECK-LABEL: @remove_shift_nuw_ne(
+; CHECK-NEXT:    [[OR:%.*]] = or i8 [[T:%.*]], [[B:%.*]]
+; CHECK-NEXT:    [[IC:%.*]] = icmp eq i8 [[OR]], 0
+; CHECK-NEXT:    ret i1 [[IC]]
+;
+  %t = shl nuw i8 %a, %s
+  %or = or i8 %t, %b
+  %ic = icmp eq i8 %or, 0
+  ret i1 %ic
+}
+
+define i1 @remove_shift_nsw_ne(i8 %a, i8 %b, i8 %s) {
+; CHECK-LABEL: @remove_shift_nsw_ne(
+; CHECK-NEXT:    [[OR:%.*]] = or i8 [[T:%.*]], [[B:%.*]]
+; CHECK-NEXT:    [[IC:%.*]] = icmp eq i8 [[OR]], 0
+; CHECK-NEXT:    ret i1 [[IC]]
+;
+  %t = shl nsw i8 %a, %s
+  %or = or i8 %t, %b
+  %ic = icmp eq i8 %or, 0
+  ret i1 %ic
+}
+
+define i1 @remove_shift_wraps(i8 %a, i8 %b, i8 %s) {
+; CHECK-LABEL: @remove_shift_wraps(
+; CHECK-NEXT:    [[T:%.*]] = shl i8 [[A:%.*]], [[S:%.*]]
+; CHECK-NEXT:    [[OR:%.*]] = or i8 [[T]], [[B:%.*]]
+; CHECK-NEXT:    [[IC:%.*]] = icmp eq i8 [[OR]], 0
+; CHECK-NEXT:    ret i1 [[IC]]
+;
+  %t = shl i8 %a, %s
+  %or = or i8 %t, %b
+  %ic = icmp eq i8 %or, 0
+  ret i1 %ic
+}
+
+define i1 @remove_shift_chain_d(i8 %a, i8 %b, i8 %c, i8 %d, i8 %s) {
+; CHECK-LABEL: @remove_shift_chain_d(
+; CHECK-NEXT:    [[OR1:%.*]] = or i8 [[A:%.*]], [[B:%.*]]
+; CHECK-NEXT:    [[OR2:%.*]] = or i8 [[C:%.*]], [[DT:%.*]]
+; CHECK-NEXT:    [[OR:%.*]] = or i8 [[OR1]], [[OR2]]
+; CHECK-NEXT:    [[IC:%.*]] = icmp eq i8 [[OR]], 0
+; CHECK-NEXT:    ret i1 [[IC]]
+;
+  %dt = shl nuw i8 %d, %s
+  %or1 = or i8 %a, %b
+  %or2 = or i8 %c, %dt
+  %or = or i8 %or1, %or2
+  %ic = icmp eq i8 %or, 0
+  ret i1 %ic
+}
+
+define i1 @remove_shift_chain_abcd(i8 %a, i8 %b, i8 %c, i8 %d, i8 %s) {
+; CHECK-LABEL: @remove_shift_chain_abcd(
+; CHECK-NEXT:    [[OR1:%.*]] = or i8 [[AT:%.*]], [[BT:%.*]]
+; CHECK-NEXT:    [[OR2:%.*]] = or i8 [[CT:%.*]], [[DT:%.*]]
+; CHECK-NEXT:    [[OR:%.*]] = or i8 [[OR1]], [[OR2]]
+; CHECK-NEXT:    [[IC:%.*]] = icmp eq i8 [[OR]], 0
+; CHECK-NEXT:    ret i1 [[IC]]
+;
+  %at = shl nuw i8 %a, %s
+  %bt = shl nuw i8 %b, 2
+  %ct = shl nuw i8 %c, 1
+  %dt = shl nuw i8 %d, %s
+  %or1 = or i8 %at, %bt
+  %or2 = or i8 %ct, %dt
+  %or = or i8 %or1, %or2
+  %ic = icmp eq i8 %or, 0
+  ret i1 %ic
+}
+

[dtcxzyw]

I am fine with this fold itself. But it prevents AggressiveInstCombine from merging consecutive loads: https://godbolt.org/z/aq94MeM3n

We should not perform this fold when the shift's LHS is zext(load).
If you think this workaround is too fragile, an alternative solution is to convert (load iN p) | (load iN p+N/8) == 0 into (load i2N p) == 0 in AggressiveInstCombine.

[davemgreen]

I've tried to move this into aggressive instcombine to allow it to check for trees of loads first. The transform is looking up through trees already, so maybe that is a better place for it. Let me know what you think.

[dtcxzyw]

LGTM. Thank you!