[llvm] r191426 - PPC: Allow partial fills in writeNopData()

[Eli Friedman]

On Thu, Sep 26, 2013 at 2:18 AM, David Majnemer <david.majnemer at gmail.com>wrote:

> Author: majnemer
> Date: Thu Sep 26 04:18:48 2013
> New Revision: 191426
>
> URL: http://llvm.org/viewvc/llvm-project?rev=191426&view=rev
> Log:
> PPC: Allow partial fills in writeNopData()
>
> When asked to pad an irregular number of bytes, we should fill with
> zeros.  This is consistent with the behavior specified in the AIX
> Assembler Language Reference as well as other LLVM and binutils
> assemblers.
>
> N.B. There is a small deviation from binutils' PPC assembler:
> when handling pads which are greater than 4 bytes but not mod 4,
> binutils will not emit any NOP sequences at all and only use zeros.
> This may or may not be a bug but there is no excellent rationale as to
> why that behavior is important to emulate.  If that behavior is needed,
> we can change writeNopData() to behave in the same way.
>
> This fixes PR17352.
>
> Modified:
>     llvm/trunk/lib/Target/PowerPC/MCTargetDesc/PPCAsmBackend.cpp
>     llvm/trunk/test/MC/PowerPC/ppc-nop.s
>
> Modified: llvm/trunk/lib/Target/PowerPC/MCTargetDesc/PPCAsmBackend.cpp
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/PowerPC/MCTargetDesc/PPCAsmBackend.cpp?rev=191426&r1=191425&r2=191426&view=diff
>
> ==============================================================================
> --- llvm/trunk/lib/Target/PowerPC/MCTargetDesc/PPCAsmBackend.cpp (original)
> +++ llvm/trunk/lib/Target/PowerPC/MCTargetDesc/PPCAsmBackend.cpp Thu Sep
> 26 04:18:48 2013
> @@ -132,14 +132,17 @@ public:
>    }
>
>    bool writeNopData(uint64_t Count, MCObjectWriter *OW) const {
> -    // Can't emit NOP with size not multiple of 32-bits
> -    if (Count % 4 != 0)
> -      return false;
> -
>      uint64_t NumNops = Count / 4;
>      for (uint64_t i = 0; i != NumNops; ++i)
>        OW->Write32(0x60000000);
>
> +    switch (Count % 4) {
> +    default: break; // No leftover bytes to write
> +    case 1: OW->Write8(0); break;
> +    case 2: OW->Write16(0); break;
> +    case 3: OW->Write16(0); OW->Write8(0); break;
> +    }
>
>
This is probably a silly question, but are you sure the switch is supposed
to be after the loop?  It looks like you're emitting misaligned nops.

-Eli
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