InstCombineCast AShr transformation

[marksl]

Summary

Transform:

%shr26 = lshr i32 %0, 16                        ; [#uses=1 type=i32]
%conv4 = trunc i32 %shr26 to i16                ; [#uses=1 type=i16]
%3 = sext i16 %conv4 to i32                     ; [#uses=N type=i32]

To:

%3 = ashr i32 %0, 16

[majnemer]

This needs a testcase.

InstCombineCasts.cpp

1258–1262

Please use more regular register names and elide the commends.

1263

I'd recommend using m_APInt so that it will match against vectors.

1267

This will explode if the ConstantInt is bigger than an i64

[marksl]

Added test case

[marksl]

Now with transform and test case.

[efriedma]

cast.ll

1443

This testcase passes on trunk. What are you trying to fix?

[marksl]

This does not appear to be an issue on the trunk.

InstCombineCasts.cpp

if (match(Src, m_AShr(m_Shl(m_Trunc(m_Value(A)), m_ConstantInt(BA)),
     unsigned MidSize = Src->getType()->getScalarSizeInBits();
     unsigned SrcDstSize = CI.getType()->getScalarSizeInBits();
     unsigned ShAmt = CA->getZExtValue()+SrcDstSize-MidSize;
     Constant *ShAmtV = ConstantInt::get(CI.getType(), ShAmt);
     A = Builder->CreateShl(A, ShAmtV, CI.getName());
     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;
 }
 
 
 /// Return a Constant* for the specified floating-point constant if it fits
 /// in the specified FP type without changing its value.
 static Constant *fitsInFPType(ConstantFP *CFP, const fltSemantics &Sem) {
   bool losesInfo;

cast.ll

 }
 
 define <2 x i32> @test90() {
 ; CHECK-LABEL: @test90(
 ; CHECK: ret <2 x i32> <i32 0, i32 15360>
   %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
+}