[PATCH] D25913: [InstCombine] Fold nuw left-shifts in `ugt`/`ule` comparisons.

[bryant via llvm-commits]

bryant updated this revision to Diff 75634.
bryant added a comment.

- Handle `uge`, `ult`.
- Prioritize over case that outputs `trunc`.


Repository:
  rL LLVM

https://reviews.llvm.org/D25913

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


Index: test/Transforms/InstCombine/icmp-shl-zext-simplify.ll
===================================================================
--- /dev/null
+++ test/Transforms/InstCombine/icmp-shl-zext-simplify.ll
@@ -0,0 +1,42 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt %s -instcombine -S | FileCheck %s
+
+define i1 @icmp_ugt_32(i64) {
+; CHECK-LABEL: @icmp_ugt_32(
+; CHECK-NEXT:    [[D:%.*]] = icmp ne i64 %0, 0
+; CHECK-NEXT:    ret i1 [[D]]
+;
+  %c = shl nuw i64 %0, 32
+  %d = icmp ugt i64 %c, 4294967295
+  ret i1 %d
+}
+
+define i1 @icmp_ule_64(i128) {
+; CHECK-LABEL: @icmp_ule_64(
+; CHECK-NEXT:    [[D:%.*]] = icmp eq i128 %0, 0
+; CHECK-NEXT:    ret i1 [[D]]
+;
+  %c = shl nuw i128 %0, 64
+  %d = icmp ule i128 %c, 18446744073709551615
+  ret i1 %d
+}
+
+define i1 @icmp_ugt_16(i64) {
+; CHECK-LABEL: @icmp_ugt_16(
+; CHECK-NEXT:    [[D:%.*]] = icmp ugt i64 %0, 15
+; CHECK-NEXT:    ret i1 [[D]]
+;
+  %c = shl nuw i64 %0, 16
+  %d = icmp ugt i64 %c, 1048575 ; 0x0f_ffff
+  ret i1 %d
+}
+
+define <2 x i1> @icmp_ule_i64x2(<2 x i64>) {
+; CHECK-LABEL: @icmp_ule_i64x2(
+; CHECK-NEXT:    [[D:%.*]] = icmp eq <2 x i64> %0, zeroinitializer
+; CHECK-NEXT:    ret <2 x i1> [[D]]
+;
+  %c = shl nuw <2 x i64> %0, <i64 16, i64 16>
+  %d = icmp ule <2 x i64> %c, <i64 65535, i64 65535>
+  ret <2 x i1> %d
+}
Index: lib/Transforms/InstCombine/InstCombineCompares.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -1950,6 +1950,30 @@
                         And, Constant::getNullValue(And->getType()));
   }
 
+  // When the shift is nuw and pred is >u or <=u, comparison only really happens
+  // in the pre-shifted bits.
+  if (Shl->hasNoUnsignedWrap()) {
+    Type *CTy = IntegerType::get(Cmp.getContext(), C->getBitWidth());
+    if (X->getType()->isVectorTy())
+      CTy = VectorType::get(CTy, X->getType()->getVectorNumElements());
+
+    APInt C_ = *C;
+    // If C is positive, pre-transform >=u/<u into >u/<=u.
+    if (C_.ugt(0)) {
+      if (Pred == ICmpInst::ICMP_UGE) {
+          C_ = C_ - 1;
+          Pred = ICmpInst::ICMP_UGT;
+      }
+      if (Pred == ICmpInst::ICMP_ULT) {
+          C_ = C_ - 1;
+          Pred = ICmpInst::ICMP_ULE;
+      }
+    }
+
+    if (Pred == ICmpInst::ICMP_UGT || Pred == ICmpInst::ICMP_ULE)
+      return new ICmpInst(Pred, X, ConstantInt::get(CTy, C_.lshr(*ShiftAmt)));
+  }
+
   // Transform (icmp pred iM (shl iM %v, N), C)
   // -> (icmp pred i(M-N) (trunc %v iM to i(M-N)), (trunc (C>>N))
   // Transform the shl to a trunc if (trunc (C>>N)) has no loss and M-N.


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