[ARM] Fix cost computation for constant hoisting

[weimingz]

Summary

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.

[weimingz]

using ">=" because when convert to signed values, getMinSignedBits() may return 65. Instead of checking both activeBits and MinSignedBits(), we just check if activeBits() is no larger than 64.

[rengolin]

Hi Weiming,

The code makes sense, but the test is not clear as to what it's all about or what's the expected output, which will make it very hard for people to follow up when it breaks for other reasons.

Can you add a comment to the test explaining what's this is about and what's the expected behaviour?

cheers,
--renato

test/Transforms/ConstantHoisting/ARM/bad-cases.ll

107

Was this line supposed to be a CHECK?

[weimingz]

Hi Renato,

Thanks for the feedback.
I Simplified the test case and added comments.

[rengolin]

LGTM, thanks!

lib/Target/ARM/ARMTargetTransformInfo.cpp

   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();

test/Transforms/ConstantHoisting/ARM/bad-cases.ll

 end:
   ret void
 }
+
+%mystruct = type { [12 x i8] }
+
+define i32 @struct_type_test(%mystruct*  %v0, %mystruct* %v1) {
+;CHECK-LABEL: @struct_type_test
+entry:
+;CHECK-NOT: %const = bitcast i96 32 to i96
+  %v2 = bitcast %mystruct* %v1 to i96*
+  %load0 = load i96, i96* %v2, align 4
+  %v3 = bitcast %mystruct* %v0 to i96*
+  %load1 = load i96, i96* %v3, align 4
+;CHECK: lshr0 = lshr i96 %load0, 32
+  %lshr0 = lshr i96 %load0, 32
+  %cast0 = trunc i96 %lshr0 to i32
+;lshr1 = lshr i96 %load1, 32
+  %lshr1 = lshr i96 %load1, 32
+  %cast1 = trunc i96 %lshr1 to i32
+  %cmp = icmp eq i32 %cast0, %cast1
+  br i1 %cmp, label %if.then, label %if.else
+
+if.then:
+  ret i32 1
+
+if.else:
+  ret i32 2;
+}