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author | Juergen Ributzka <juergen@apple.com> | 2014-01-24 18:23:08 +0000 |
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committer | Juergen Ributzka <juergen@apple.com> | 2014-01-24 18:23:08 +0000 |
commit | fb282c68b7a31e8ee2810c81f5a6ed06e77cf7d1 (patch) | |
tree | 6b63d9b1a0642aff1c8bc52a62876348e27588bb /include/llvm/Analysis/TargetTransformInfo.h | |
parent | 8346f147ab6f06be4dac4af5c0e451a22bccf475 (diff) | |
download | llvm-fb282c68b7a31e8ee2810c81f5a6ed06e77cf7d1.tar.gz llvm-fb282c68b7a31e8ee2810c81f5a6ed06e77cf7d1.tar.bz2 llvm-fb282c68b7a31e8ee2810c81f5a6ed06e77cf7d1.tar.xz |
Add Constant Hoisting Pass
This pass identifies expensive constants to hoist and coalesces them to
better prepare it for SelectionDAG-based code generation. This works around the
limitations of the basic-block-at-a-time approach.
First it scans all instructions for integer constants and calculates its
cost. If the constant can be folded into the instruction (the cost is
TCC_Free) or the cost is just a simple operation (TCC_BASIC), then we don't
consider it expensive and leave it alone. This is the default behavior and
the default implementation of getIntImmCost will always return TCC_Free.
If the cost is more than TCC_BASIC, then the integer constant can't be folded
into the instruction and it might be beneficial to hoist the constant.
Similar constants are coalesced to reduce register pressure and
materialization code.
When a constant is hoisted, it is also hidden behind a bitcast to force it to
be live-out of the basic block. Otherwise the constant would be just
duplicated and each basic block would have its own copy in the SelectionDAG.
The SelectionDAG recognizes such constants as opaque and doesn't perform
certain transformations on them, which would create a new expensive constant.
This optimization is only applied to integer constants in instructions and
simple (this means not nested) constant cast experessions. For example:
%0 = load i64* inttoptr (i64 big_constant to i64*)
Reviewed by Eric
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200022 91177308-0d34-0410-b5e6-96231b3b80d8
Diffstat (limited to 'include/llvm/Analysis/TargetTransformInfo.h')
-rw-r--r-- | include/llvm/Analysis/TargetTransformInfo.h | 8 |
1 files changed, 8 insertions, 0 deletions
diff --git a/include/llvm/Analysis/TargetTransformInfo.h b/include/llvm/Analysis/TargetTransformInfo.h index a8975b7a4f..f2613e35c5 100644 --- a/include/llvm/Analysis/TargetTransformInfo.h +++ b/include/llvm/Analysis/TargetTransformInfo.h @@ -92,6 +92,7 @@ public: enum TargetCostConstants { TCC_Free = 0, ///< Expected to fold away in lowering. TCC_Basic = 1, ///< The cost of a typical 'add' instruction. + TCC_Load = 3, TCC_Expensive = 4 ///< The cost of a 'div' instruction on x86. }; @@ -299,6 +300,13 @@ public: /// immediate of the specified type. virtual unsigned getIntImmCost(const APInt &Imm, Type *Ty) const; + /// \brief Return the expected cost of materialization for the given integer + /// immediate of the specified type for a given instruction. The cost can be + /// zero if the immediate can be folded into the specified instruction. + virtual unsigned getIntImmCost(unsigned Opcode, const APInt &Imm, + Type *Ty) const; + virtual unsigned getIntImmCost(Intrinsic::ID IID, const APInt &Imm, + Type *Ty) const; /// @} /// \name Vector Target Information |