[Mesa-dev] [PATCH] tgsi: document and fixup IF and BREAKC instrunctions

[Roland Scheidegger]

Everything emitting those opcodes always assumed this is some sort of
boolean until now, so treating them as floats always was sketchy. Note
that the differences to treating them as uints or floats for comparisons
to zero isn't large, it's the same for everything except -0.0 and NaNs,
and I suspect hw which doesn't have integers doesn't have NaNs neither.
Which would leave the -0.0 case but unless you happened to encode
results of comparisons as -0.0 there should not be a problem neither -
obviously this would not work for state trackers which really require
the input be treated as uint but that should only ever affect things
which require integers anyway I think.

Roland

Am 11.04.2013 21:31, schrieb Marek Olšák:
> What about drivers without integer support? The IF instruction should
> have 2 definitions: one for the drivers which support
> PIPE_SHADER_CAP_INTEGERS, and the other one for those which don't.
> Obviously, there is no way to change the behavior of the IF opcode if
> you only have floats.
> 
> Marek
> 
> 
> On Thu, Apr 11, 2013 at 5:20 AM, Zack Rusin <zackr at vmware.com
> <mailto:zackr at vmware.com>> wrote:
> 
>     As implemented in tgsi_exec they both test src operands on the bit
>     level and don't do floating point comperisons as some thought.
>     This documents them as such to avoid future confusion and fixes
>     their implementation in llvmpipe.
> 
>     Could gallium driver developers make sure that they're handling
>     those instrunctions correctly? From my quick glance it seems
>     to be ok, but I don't know those codebases at all.
> 
>     Signed-off-by: Zack Rusin <zackr at vmware.com <mailto:zackr at vmware.com>>
>     ---
>      src/gallium/auxiliary/gallivm/lp_bld_tgsi_soa.c |   10 ++++------
>      src/gallium/auxiliary/tgsi/tgsi_info.c          |    2 ++
>      src/gallium/docs/source/tgsi.rst                |   16 ++++++++++------
>      3 files changed, 16 insertions(+), 12 deletions(-)
> 
>     diff --git a/src/gallium/auxiliary/gallivm/lp_bld_tgsi_soa.c
>     b/src/gallium/auxiliary/gallivm/lp_bld_tgsi_soa.c
>     index 853de09..8a29635 100644
>     --- a/src/gallium/auxiliary/gallivm/lp_bld_tgsi_soa.c
>     +++ b/src/gallium/auxiliary/gallivm/lp_bld_tgsi_soa.c
>     @@ -2370,12 +2370,9 @@ breakc_emit(
>         struct lp_build_emit_data * emit_data)
>      {
>         struct lp_build_tgsi_soa_context * bld = lp_soa_context(bld_base);
>     -   LLVMBuilderRef builder = bld_base->base.gallivm->builder;
>         struct lp_build_context *uint_bld = &bld_base->uint_bld;
>     -   LLVMValueRef unsigned_cond =
>     -      LLVMBuildBitCast(builder, emit_data->args[0],
>     uint_bld->vec_type, "");
>         LLVMValueRef cond = lp_build_cmp(uint_bld, PIPE_FUNC_NOTEQUAL,
>     -                                    unsigned_cond,
>     +                                    emit_data->args[0],
>                                          uint_bld->zero);
> 
>         lp_exec_break_condition(&bld->exec_mask, cond);
>     @@ -2389,9 +2386,10 @@ if_emit(
>      {
>         LLVMValueRef tmp;
>         struct lp_build_tgsi_soa_context * bld = lp_soa_context(bld_base);
>     +   struct lp_build_context *uint_bld = &bld_base->uint_bld;
> 
>     -   tmp = lp_build_cmp(&bld_base->base, PIPE_FUNC_NOTEQUAL,
>     -                      emit_data->args[0], bld->bld_base.base.zero);
>     +   tmp = lp_build_cmp(uint_bld, PIPE_FUNC_NOTEQUAL,
>     +                      emit_data->args[0], uint_bld->zero);
>         lp_exec_mask_cond_push(&bld->exec_mask, tmp);
>      }
> 
>     diff --git a/src/gallium/auxiliary/tgsi/tgsi_info.c
>     b/src/gallium/auxiliary/tgsi/tgsi_info.c
>     index 1fadfec..f488351 100644
>     --- a/src/gallium/auxiliary/tgsi/tgsi_info.c
>     +++ b/src/gallium/auxiliary/tgsi/tgsi_info.c
>     @@ -297,6 +297,8 @@ tgsi_opcode_infer_src_type( uint opcode )
>         case TGSI_OPCODE_TXF:
>         case TGSI_OPCODE_SAMPLE_I:
>         case TGSI_OPCODE_SAMPLE_I_MS:
>     +   case TGSI_OPCODE_IF:
>     +   case TGSI_OPCODE_BREAKC:
>            return TGSI_TYPE_UNSIGNED;
>         case TGSI_OPCODE_MOD:
>         case TGSI_OPCODE_I2F:
>     diff --git a/src/gallium/docs/source/tgsi.rst
>     b/src/gallium/docs/source/tgsi.rst
>     index 28308cb..e6d97d6 100644
>     --- a/src/gallium/docs/source/tgsi.rst
>     +++ b/src/gallium/docs/source/tgsi.rst
>     @@ -863,10 +863,19 @@ This instruction replicates its result.
> 
>        TBD
> 
>     +
>     +.. opcode:: BREAKC - Break Conditional
>     +
>     +  Conditionally moves the point of execution to the instruction
>     after the
>     +  next endloop or endswitch. The instruction must appear within a
>     loop/endloop
>     +  or switch/endswitch. The 32-bit register supplied by src0 is
>     tested at a
>     +  bit level (treat it as unsigned).
>     +
> 
>      .. opcode:: IF - If
> 
>     -  TBD
>     +  Branch based on logical OR result. The 32-bit register supplied
>     by src0 is
>     +  tested at a bit level. If any bit is nonzero, it will evaluate to
>     true.
> 
> 
>      .. opcode:: ELSE - Else
>     @@ -1202,11 +1211,6 @@ XXX wait what
> 
>        TBD
> 
>     -
>     -.. opcode:: BREAKC - Break Conditional
>     -
>     -  TBD
>     -
>      .. _doubleopcodes:
> 
>      Double ISA
>     --
>     1.7.10.4
> 
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> 
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