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[libreriscv.git] / simple_v_extension.mdwn

diff --git a/simple_v_extension.mdwn b/simple_v_extension.mdwn
index 99ae03031cdf81bf80a89a6d9e58dd98027effcb..4cf0937edde233618bbd012a4a8ad571c9acd473 100644 (file)
--- a/simple_v_extension.mdwn
+++ b/simple_v_extension.mdwn
@@ -1,14 +1,5 @@
 # Variable-width Variable-packed SIMD / Simple-V / Parallelism Extension Proposal
 
-* TODO 23may2018: CSR-CAM-ify regfile tables
-* TODO 23may2018: zero-mark predication CSR
-* TODO 28may2018: sort out VSETVL: CSR length to be removed?
-* TODO 09jun2018: Chennai Presentation more up-to-date
-* TODO 09jun2019: elwidth only 4 values (dflt, dflt/2, 8, 16)
-* TODO 09jun2019: extra register banks (future option)
-* TODO 09jun2019: new Reg CSR table (incl. packed=Y/N)
-
-
 Key insight: Simple-V is intended as an abstraction layer to provide
 a consistent "API" to parallelisation of existing *and future* operations.
 *Actual* internal hardware-level parallelism is *not* required, such
@@ -18,8 +9,8 @@ instruction queue (FIFO), pending execution.
 
 *Actual* parallelism, if added independently of Simple-V in the form
 of Out-of-order restructuring (including parallel ALU lanes) or VLIW
-implementations, or SIMD, or anything else, would then benefit *if*
-Simple-V was added on top.
+implementations, or SIMD, or anything else, would then benefit from
+the uniformity of a consistent API.
 
 [[!toc ]]
 
@@ -1068,7 +1059,7 @@ Similar rules apply to the destination register.
 * Throw an exception.  Whether that actually results in spawning threads
   as part of the trap-handling remains to be seen.
 
-# Under consideration
+# Under consideration <a name="issues"></a>
 
 From the Chennai 2018 slides the following issues were raised.
 Efforts to analyse and answer these questions are below.
@@ -1191,7 +1182,44 @@ It is quite complex, in other words, and needs careful consideration.
 
 ## 8/16-bit ops is it worthwhile adding a "start offset"?
 
-TBD
+The idea here is to make it possible, particularly in a "Packed SIMD"
+case, to be able to avoid doing unaligned Load/Store operations
+by specifying that operations, instead of being carried out
+element-for-element, are offset by a fixed amount *even* in 8 and 16-bit
+element Packed SIMD cases.
+
+For example rather than take 2 32-bit registers divided into 4 8-bit
+elements and have them ADDed element-for-element as follows:
+
+    r3[0] = add r4[0], r6[0]
+    r3[1] = add r4[1], r6[1]
+    r3[2] = add r4[2], r6[2]
+    r3[3] = add r4[3], r6[3]
+
+an offset of 1 would result in four operations as follows, instead:
+
+    r3[0] = add r4[1], r6[0]
+    r3[1] = add r4[2], r6[1]
+    r3[2] = add r4[3], r6[2]
+    r3[3] = add r5[0], r6[3]
+
+In non-packed-SIMD mode there is no benefit at all, as a vector may
+be created using a different CSR that has the offset built-in.  So this
+leaves just the packed-SIMD case to consider.
+
+Two ways in which this could be implemented / emulated (without special
+hardware):
+
+* bit-manipulation that shuffles the data along by one byte (or one word)
+  either prior to or as part of the operation requiring the offset.
+* just use an unaligned Load/Store sequence, even if there are performance
+  penalties for doing so.
+
+The question then is whether the performance hit is worth the extra hardware
+involving byte-shuffling/shifting the data by an arbitrary offset.  On
+balance given that there are two reasonable instruction-based options, the
+hardware-offset option should be left out for the initial version of SV,
+with the option to consider it in an "advanced" version of the specification.
 
 # Impementing V on top of Simple-V