Data-dependent fail-on-first has two distinct variants: one for LD/ST, the other for arithmetic operations (actually, CR-driven). Note in each case the assumption is that vector elements are required appear to be executed in sequential Program Order, element 0 being the first.
* LD/ST ffirst treats the first LD/ST in a vector (element 0) as an ordinary one. Exceptions occur "as normal". However for elements 1 and above, if an exception would occur, then VL is **truncated** to the previous element.
-* Data-driven (CR-driven) fail-on-first activates when Rc=1 or other CR-creating operation produces a result (including cmp). Similar to branch, an analysis of the CR is performed and if the test fails, the vector operation terminates and discards all element operations at and above the current one, and VL is truncated to the *previous* element. Thus the new VL comprises a vector of results that pass certain criteria (equal to zero, less than zero).
+* Data-driven (CR-driven) fail-on-first activates when Rc=1 or other CR-creating operation produces a result (including cmp). Similar to branch, an analysis of the CR is performed and if the test fails, the vector operation terminates and discards all element operations at and above the current one, and VL is truncated to the *previous* element. Thus the new VL comprises a contiguous vector of results, all of which pass the testing criteria (equal to zero, less than zero).
The CR-based data-driven fail-on-first is new and not found in ARM SVE or RVV. It is extremely useful for reducing instruction count, however requires speculative execution involving modifications of VL to get high performance implementations.
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