[ARM] Fix 28282: cost computation for constant hoisting

Summary:
This fixes bug: https://llvm.org/bugs/show_bug.cgi?id=28282

Currently the cost model of constant hoisting checks the bit width of the data type of the constants.
However, the actual immediate value is small enough and not need to be hoisted.
This patch checks for the actual bit width needed for the constant.

Reviewers: t.p.northover, rengolin

Subscribers: aemerson, rengolin, llvm-commits

Differential Revision: http://reviews.llvm.org/D21668

llvm-svn: 274073

2 files changed, 18 insertions, 1 deletions

diff --git a/llvm/lib/Target/ARM/ARMTargetTransformInfo.cpp b/llvm/lib/Target/ARM/ARMTargetTransformInfo.cpp
index aaec3107aec..ddb5d287780 100644
--- a/llvm/lib/Target/ARM/ARMTargetTransformInfo.cpp
+++ b/llvm/lib/Target/ARM/ARMTargetTransformInfo.cpp
@@ -19,7 +19,7 @@ int ARMTTIImpl::getIntImmCost(const APInt &Imm, Type *Ty) {
   assert(Ty->isIntegerTy());
 
  unsigned Bits = Ty->getPrimitiveSizeInBits();
- if (Bits == 0 || Bits > 64)
+ if (Bits == 0 || Imm.getActiveBits() >= 64)
    return 4;
 
   int64_t SImmVal = Imm.getSExtValue();
diff --git a/llvm/test/Transforms/ConstantHoisting/ARM/bad-cases.ll b/llvm/test/Transforms/ConstantHoisting/ARM/bad-cases.ll
index 8fa78e3d69b..ffcfb2e56c9 100644
--- a/llvm/test/Transforms/ConstantHoisting/ARM/bad-cases.ll
+++ b/llvm/test/Transforms/ConstantHoisting/ARM/bad-cases.ll
@@ -90,3 +90,20 @@ loop:
 end:
   ret void
 }
+
+;PR 28282: even when data type is larger than 64-bit, the bit width of the
+;constant operand could be smaller than 64-bit. In this case, there is no
+;benefit to hoist the constant.
+define i32 @struct_type_test(i96 %a0, i96 %a1) {
+;CHECK-LABEL: @struct_type_test
+entry:
+;CHECK-NOT: %const = bitcast i96 32 to i96
+;CHECK: lshr0 = lshr i96 %a0, 32
+  %lshr0 = lshr i96 %a0, 32
+  %cast0 = trunc i96 %lshr0 to i32
+;CHECK: lshr1 = lshr i96 %a1, 32
+  %lshr1 = lshr i96 %a1, 32
+  %cast1 = trunc i96 %lshr1 to i32
+  %ret = add i32 %cast0, %cast1
+  ret i32 %ret
+}