[llvm] r235679 - Improve isTriviallyReMaterializable() documentation.

[Quentin Colombet]

For the record, this is a follow-up of r235673 that we discussed offline.

Thanks Matthias,
Quentin
> On Apr 23, 2015, at 6:15 PM, Matthias Braun <matze at braunis.de> wrote:
> 
> Author: matze
> Date: Thu Apr 23 20:15:27 2015
> New Revision: 235679
> 
> URL: http://llvm.org/viewvc/llvm-project?rev=235679&view=rev
> Log:
> Improve isTriviallyReMaterializable() documentation.
> 
> This should make it clear under which narrow circumstances implicit
> physreg uses are okay when rematerializing and prevent people from
> accidentally allowing too much when overriding
> isReallyTriviallyReMaterializable() even with the weaker assert in the
> RegisterCoalescer.
> 
> Modified:
>    llvm/trunk/include/llvm/Target/TargetInstrInfo.h
> 
> Modified: llvm/trunk/include/llvm/Target/TargetInstrInfo.h
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/Target/TargetInstrInfo.h?rev=235679&r1=235678&r2=235679&view=diff
> ==============================================================================
> --- llvm/trunk/include/llvm/Target/TargetInstrInfo.h (original)
> +++ llvm/trunk/include/llvm/Target/TargetInstrInfo.h Thu Apr 23 20:15:27 2015
> @@ -67,9 +67,11 @@ public:
>                                          const TargetRegisterInfo *TRI,
>                                          const MachineFunction &MF) const;
> 
> -  /// isTriviallyReMaterializable - Return true if the instruction is trivially
> -  /// rematerializable, meaning it has no side effects and requires no operands
> -  /// that aren't always available.
> +  /// Return true if the instruction is trivially rematerializable, meaning it
> +  /// has no side effects and requires no operands that aren't always available.
> +  /// This means the only allowed uses are constants and unallocatable physical
> +  /// registers so that the instructions result is independent of the place
> +  /// in the function.
>   bool isTriviallyReMaterializable(const MachineInstr *MI,
>                                    AliasAnalysis *AA = nullptr) const {
>     return MI->getOpcode() == TargetOpcode::IMPLICIT_DEF ||
> @@ -79,12 +81,13 @@ public:
>   }
> 
> protected:
> -  /// isReallyTriviallyReMaterializable - For instructions with opcodes for
> -  /// which the M_REMATERIALIZABLE flag is set, this hook lets the target
> -  /// specify whether the instruction is actually trivially rematerializable,
> -  /// taking into consideration its operands. This predicate must return false
> -  /// if the instruction has any side effects other than producing a value, or
> -  /// if it requres any address registers that are not always available.
> +  /// For instructions with opcodes for which the M_REMATERIALIZABLE flag is
> +  /// set, this hook lets the target specify whether the instruction is actually
> +  /// trivially rematerializable, taking into consideration its operands. This
> +  /// predicate must return false if the instruction has any side effects other
> +  /// than producing a value, or if it requres any address registers that are
> +  /// not always available.
> +  /// Requirements must be check as stated in isTriviallyReMaterializable() .
>   virtual bool isReallyTriviallyReMaterializable(const MachineInstr *MI,
>                                                  AliasAnalysis *AA) const {
>     return false;
> 
> 
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