[PATCH] D38521: [InstCombine] Improve support for ashr in foldICmpAndShift

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This revision was automatically updated to reflect the committed changes.
Closed by commit rL314945: [InstCombine] Improve support for ashr in foldICmpAndShift (authored by ctopper).

Changed prior to commit:
  https://reviews.llvm.org/D38521?vs=117600&id=117756#toc

Repository:
  rL LLVM

https://reviews.llvm.org/D38521

Files:
  llvm/trunk/lib/Transforms/InstCombine/InstCombineCompares.cpp
  llvm/trunk/test/Transforms/InstCombine/icmp.ll


Index: llvm/trunk/lib/Transforms/InstCombine/InstCombineCompares.cpp
===================================================================
--- llvm/trunk/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ llvm/trunk/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -1524,25 +1524,28 @@
   const APInt *C3;
   if (match(Shift->getOperand(1), m_APInt(C3))) {
     bool CanFold = false;
-    if (ShiftOpcode == Instruction::AShr) {
-      // There may be some constraints that make this possible, but nothing
-      // simple has been discovered yet.
-      CanFold = false;
-    } else if (ShiftOpcode == Instruction::Shl) {
+    if (ShiftOpcode == Instruction::Shl) {
       // For a left shift, we can fold if the comparison is not signed. We can
       // also fold a signed comparison if the mask value and comparison value
       // are not negative. These constraints may not be obvious, but we can
       // prove that they are correct using an SMT solver.
       if (!Cmp.isSigned() || (!C2.isNegative() && !C1.isNegative()))
         CanFold = true;
-    } else if (ShiftOpcode == Instruction::LShr) {
+    } else {
+      bool IsAshr = ShiftOpcode == Instruction::AShr;
       // For a logical right shift, we can fold if the comparison is not signed.
       // We can also fold a signed comparison if the shifted mask value and the
       // shifted comparison value are not negative. These constraints may not be
       // obvious, but we can prove that they are correct using an SMT solver.
-      if (!Cmp.isSigned() ||
-          (!C2.shl(*C3).isNegative() && !C1.shl(*C3).isNegative()))
-        CanFold = true;
+      // For an arithmetic shift right we can do the same, if we ensure
+      // the And doesn't use any bits being shifted in. Normally these would
+      // be turned into lshr by SimplifyDemandedBits, but not if there is an
+      // additional user.
+      if (!IsAshr || (C2.shl(*C3).lshr(*C3) == C2)) {
+        if (!Cmp.isSigned() ||
+            (!C2.shl(*C3).isNegative() && !C1.shl(*C3).isNegative()))
+          CanFold = true;
+      }
     }
 
     if (CanFold) {
Index: llvm/trunk/test/Transforms/InstCombine/icmp.ll
===================================================================
--- llvm/trunk/test/Transforms/InstCombine/icmp.ll
+++ llvm/trunk/test/Transforms/InstCombine/icmp.ll
@@ -1634,6 +1634,50 @@
   ret i1 %and3
 }
 
+; Variation of the above with an ashr
+define i1 @icmp_and_ashr_multiuse(i32 %X) {
+; CHECK-LABEL: @icmp_and_ashr_multiuse(
+; CHECK-NEXT:    [[AND:%.*]] = and i32 [[X:%.*]], 240
+; CHECK-NEXT:    [[AND2:%.*]] = and i32 [[X]], 496
+; CHECK-NEXT:    [[TOBOOL:%.*]] = icmp ne i32 [[AND]], 224
+; CHECK-NEXT:    [[TOBOOL2:%.*]] = icmp ne i32 [[AND2]], 432
+; CHECK-NEXT:    [[AND3:%.*]] = and i1 [[TOBOOL]], [[TOBOOL2]]
+; CHECK-NEXT:    ret i1 [[AND3]]
+;
+  %shr = ashr i32 %X, 4
+  %and = and i32 %shr, 15
+  %and2 = and i32 %shr, 31 ; second use of the shift
+  %tobool = icmp ne i32 %and, 14
+  %tobool2 = icmp ne i32 %and2, 27
+  %and3 = and i1 %tobool, %tobool2
+  ret i1 %and3
+}
+
+define i1 @icmp_lshr_and_overshift(i8 %X) {
+; CHECK-LABEL: @icmp_lshr_and_overshift(
+; CHECK-NEXT:    [[TOBOOL:%.*]] = icmp ugt i8 [[X:%.*]], 31
+; CHECK-NEXT:    ret i1 [[TOBOOL]]
+;
+  %shr = lshr i8 %X, 5
+  %and = and i8 %shr, 15
+  %tobool = icmp ne i8 %and, 0
+  ret i1 %tobool
+}
+
+; We shouldn't simplify this because the and uses bits that are shifted in.
+define i1 @icmp_ashr_and_overshift(i8 %X) {
+; CHECK-LABEL: @icmp_ashr_and_overshift(
+; CHECK-NEXT:    [[SHR:%.*]] = ashr i8 [[X:%.*]], 5
+; CHECK-NEXT:    [[AND:%.*]] = and i8 [[SHR]], 15
+; CHECK-NEXT:    [[TOBOOL:%.*]] = icmp ne i8 [[AND]], 0
+; CHECK-NEXT:    ret i1 [[TOBOOL]]
+;
+  %shr = ashr i8 %X, 5
+  %and = and i8 %shr, 15
+  %tobool = icmp ne i8 %and, 0
+  ret i1 %tobool
+}
+
 ; PR16244
 define i1 @test71(i8* %x) {
 ; CHECK-LABEL: @test71(


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