gallium/tgsi: Rewrite the docs on source modifiers.
Clarify what the rules are, and warn about the exceptions. Reviewed-by: Marek Olšák <marek.olsak@amd.com> Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8369>
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@ -23,26 +23,29 @@ When an instruction has a scalar result, the result is usually copied into
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each of the components of *dst*. When this happens, the result is said to be
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*replicated* to *dst*. :opcode:`RCP` is one such instruction.
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Modifiers
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Source Modifiers
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^^^^^^^^^^^^^^^^
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TGSI supports 32-bit negate and absolute value modifiers on floating-point
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inputs, and 32-bit integer negates on some drivers. The negate applies after
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absolute value if both are present.
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The type of an input can be found by ``tgsi_opcode_infer_src_type()``, and
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TGSI_OPCODE_MOV and the second and third operands of TGSI_OPCODE_UCMP (which
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return TGSI_TYPE_UNTYPED) are also considered floats for the purpose of source
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modifiers.
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Other Modifiers
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^^^^^^^^^^^^^^^
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TGSI supports modifiers on inputs (as well as saturate and precise modifier
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on instructions).
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The saturate modifier clamps 32-bit destination stores to [0.0, 1.0].
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For arithmetic instruction having a precise modifier certain optimizations
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which may alter the result are disallowed. Example: *add(mul(a,b),c)* can't be
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optimized to TGSI_OPCODE_MAD, because some hardware only supports the fused
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MAD instruction.
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For inputs which have a floating point type, both absolute value and
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negation modifiers are supported (with absolute value being applied
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first). The only source of TGSI_OPCODE_MOV and the second and third
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sources of TGSI_OPCODE_UCMP are considered to have float type for
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applying modifiers.
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For inputs which have signed or unsigned type only the negate modifier is
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supported.
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Instruction Set
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---------------
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