Agner`s CPU blog

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Merging with first operand
Author: Hubert Lamontagne Date: 2016-08-19 17:52
Agner wrote:
If you have enabled exceptions for integer overflow and you want to avoid the hypothetical overflow exception in a not-taken branch then you have to put the mask on the addition instuction as well. In other words, you will have two masked instructions using the same mask.

This is the reason why exceptions are not good in a vector processor. In floating point code you can disable exceptions and rely on the propagation of NAN and INF values to detect numeric errors. But there is no easy way of detecting integer overflow. I have discussed this in a previous post, but there seems to be no good solution. Many compilers and programming languages simply have no easy way of detecting integer overflow, but this may be a security problem, so I thought that we must provide some method. The following methods for detecting integer overflow may be considered:

  1. Use a flag output. This gives every ALU instruction an extra output dependency, and also an extra input dependency if you want the overflow bit to be propagated through a series of calculations. The extra input and output dependencies hamper out-of-order processing.
  2. Add an extra overflow bit for each vector element in a vector register. This adds an extra complication for saving the overflow bits when a vector register is spilled to memory.
  3. Use exception traps (interrupts) to catch integer overflow. This requires a minimum of complexity to the instruction set, but it has a problem with vector code.
  4. Use the even-numbered elements in a vector register for calculation and the odd-numbered elements for propagating overflow information. This is relatively easy to implement but wasteful because only half of the ALUs are used.
Method 1 has been rejected. The other three methods are still being considered for use in ForwardCom.
I can think of a couple more options:

Method 5 : Have an extra instruction that applies the operation but generates potential-overflow-or-exception flags instead of result values. This is equivalent to option 1 but using 2 instructions instead of one (but is roughly the same cost in the pipeline since you need 2 micro-ops in both case to store all the results), or it's like option 4 but without having the two results interlaced together.

Method 6 : Have separate vector flags registers. This is like method 1 but instead of having the extra inputs and outputs go to the same register file (which requires more ports and thus more micro-ops), they go to a separate register file that only deals with flags which means that they can have their own register renamer and have their own ports, which avoids the N^2 growth associated with adding more ports. Plus, you can mandate that the whole flags register is always updated to avoid x86 flags register merging insanity. And you can probably have a limit of 1 op per cycle on the number of flags operations to limit the size of the extra flags manipulation block you'd be adding to the CPU (see how ARM did this: most arithmetic ops never touch the flags so it needs much less aggressive handling of flags). If you're doing heavy duty flags register operations, this might be worth the trouble - which is why 8bit and 16bit CPUs all have flags (they need it to deal with oversized integers).

Method 7 : Do it all in software. This makes checked operations take multiple instructions which might be slower. But it reduces the overall complexity of the design which means that you might be able to run 1 or 2 more instructions per cycle, which probably offsets the extra cost of separate checking. This can probably be combined with option 5 (just have a couple extra instructions for speeding up overflow checking for the cases that really need it).

Very rough cost-benefit analysis:
1: Requires 2 micro-ops. Note that this is fine if you have to support 2 micro-op operations for other reasons.
2: Effects depends on the ABI. If you mandate that callees don't have to save the flags and can trash them, then you only need special save/restore in interrupt handlers.
3: Depends on if the design is in-order or out-of-order. For in-order, this makes an Arm NEON style delayed-SIMD pipeline impossible because all downstream instructions become conditional. On out-of-order, this adds an extra path from the SIMD unit to the retirement unit to signal potential interrupts (plus more complex conditional writeback of other operations retiring on the same cycle).
4: Potentially adds more shuffling of values around to add and remove space for flags. Plus, you're potentially getting half as much vector processing for a given register size.
5: Adds more ALU instructions to deal with. Added instructions are kinda quirky.
6: Requires addition of full vector flags handling unit, some instructions to load/store flag reg values, potential stalls if too many flags instructions are issued at the same time.
7: Error checked code that isn't limited by memory or branch prediction might run slower, but non-error-checked code might run faster. Simpler architecture is easier to implement and verify.

 
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