[PATCH] D32240: InstCombineCast AShr transformation

[Mark Schimmel via Phabricator via llvm-commits]

marksl updated this revision to Diff 95834.
marksl added a comment.

Now with transform and test case.


https://reviews.llvm.org/D32240

Files:
  InstCombineCasts.cpp
  cast.ll


Index: cast.ll
===================================================================
--- cast.ll
+++ cast.ll
@@ -1432,3 +1432,13 @@
   %tmp6 = bitcast <4 x half> <half undef, half undef, half undef, half 0xH3C00> to <2 x i32>
   ret <2 x i32> %tmp6
 }
+
+define i32 @test91(i32 %i) {
+; CHECK-LABEL: @test91(
+; CHECK: ashr
+; CHECK: ret
+  %a = lshr i32 %i, 16
+  %b = trunc i32 %a to i16
+  %c = sext i16 %b to i32
+  ret i32 %c
+}
Index: InstCombineCasts.cpp
===================================================================
--- InstCombineCasts.cpp
+++ InstCombineCasts.cpp
@@ -1254,6 +1254,21 @@
     return BinaryOperator::CreateAShr(A, ShAmtV);
   }
 
+  // Transform:
+  //   %a = lshr i32 %i, 16
+  //   %b = trunc i32 %a to i16
+  //   %c = sext i16 %b to i32
+  // To:
+  //   %c = ashr i32 %i, 16
+  const APInt *SA = nullptr;
+  if (match(Src, m_OneUse(m_Trunc( m_OneUse(m_LShr(m_Value(A), m_APInt(SA)))))) &&
+      A->getType() == CI.getType()) {
+    unsigned SextSize = CI.getType()->getScalarSizeInBits();
+    unsigned TruncSize = Src->getType()->getScalarSizeInBits();
+    if (*SA == SextSize - TruncSize)
+      return BinaryOperator::CreateAShr(A, ConstantInt::get(CI.getContext(), *SA));
+  }
+
   return nullptr;
 }
 


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