[PATCH] D62818: [InstCombine] Allow ((X << Y) & SignMask) != 0 to be optimized as (X << Y) s< 0.

[Huihui Zhang via Phabricator via llvm-commits]

huihuiz created this revision.
huihuiz added reviewers: spatel, craig.topper, efriedma.
Herald added a project: LLVM.

For input ICmp instruction ((X << Y) & C) == 0, if C is sign mask,
do not convert this into (X & (C >> Y)) == 0.


Repository:
  rL LLVM

https://reviews.llvm.org/D62818

Files:
  lib/Transforms/InstCombine/InstCombineCompares.cpp
  test/Transforms/InstCombine/icmp-shl-and.ll


Index: test/Transforms/InstCombine/icmp-shl-and.ll
===================================================================
--- /dev/null
+++ test/Transforms/InstCombine/icmp-shl-and.ll
@@ -0,0 +1,17 @@
+; RUN: opt %s -instcombine -S | FileCheck %s
+
+; CHECK-LABEL: foo
+; CHECK: [[SUB:%.*]] = add nsw i32 %x, -1
+; CHECK-NEXT: [[SHL:%.*]] = shl i32 %y, [[SUB]]
+; CHECK-NEXT  icmp slt i32 [[SHL]], 0
+
+define dso_local i32 @foo(i32 %x, i32 %y, i32 %a, i32 %b) {
+entry:
+  %sub = sub nsw i32 %x, 1
+  %shl = shl i32 %y, %sub
+  %and = and i32 %shl, -2147483648
+  %tobool = icmp ne i32 %and, 0
+  %selv = select i1 %tobool, i32 %a, i32 %b
+  ret i32 %selv
+}
+
Index: lib/Transforms/InstCombine/InstCombineCompares.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -1607,7 +1607,8 @@
   // preferable because it allows the C2 << Y expression to be hoisted out of a
   // loop if Y is invariant and X is not.
   if (Shift->hasOneUse() && C1.isNullValue() && Cmp.isEquality() &&
-      !Shift->isArithmeticShift() && !isa<Constant>(Shift->getOperand(0))) {
+      !C2.isSignMask() && !Shift->isArithmeticShift() &&
+      !isa<Constant>(Shift->getOperand(0))) {
     // Compute C2 << Y.
     Value *NewShift =
         IsShl ? Builder.CreateLShr(And->getOperand(1), Shift->getOperand(1))


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