What a shame for clang and gcc! They still don't implement perfect hashes for large sparse switch tables. I did that recently for my rcc C compiler, and it was just a single day of work. Esp. beneficial for unicode work. The LLVM patch was rejected a decade ago. Gcc has several unicode patches sitting in branches, which would also need that
Maybe LLVM doesn't have time for performance improvements due to still reading through 30000 open issues, which also include miscompilations leading to incorrect results at runtime?
For example, my bug report that `+= 2` is broken has not been acknowledged (read?) by anyone for 3 years:
const size_t n = 13; // triggers the issue
#pragma omp parallel for
for (uint32_t i = 0; i < n; i += 2) { // counts PAST 13!
Would like to hear more about the performance characteristics. Do you perfect hash to a contiguous range and then use a jump table, or do you perfect hash directly to relative offsets to basic blocks?
> The switch in sparse_8 could be simplified by dividing x by eight before the jump. Interestingly, gcc doesn't find this simplification.
It could not because that would ensure that 7 values for each case would be handled by function calls instead of the default case - 0/8 is 0, but 3/8 is also 0
Okay. Good point, that is quite a bit shorter than I first thought of
A rough benchmark of a version of that against the original shows it to be close to twice as fast for unpredictable input with GCC (4.5 ns vs 2.72 ns) on my old machine. With predictable input where x values often are the same the original version is quicker
int sparse_8_jump(int x, int val) {
unsigned int idx = (unsigned int)x >> 3;
if (idx > 5)
return 0;
if (x & 7)
return 0;
switch (idx) {
case 0:
return f0(val);
case 1:
return f1(val);
case 2:
return f2(val);
case 3:
return f3(val);
case 5:
return f4(val);
default:
return 0;
}
}
Reducing the range would have been useful to reduce the size of a jump table, but when you have a decision tree then it does not matter.
... unless the size of immediate parameters would have affected code size. In the example code however, all constants fit in an `imm8` parameter to `cmp`.
Each `jmp` to f0..fX is an optimisation of a `call` and a `ret`, and probably done in a separate pass after the switch-case code has already been expanded.
The `mov edi, esi` at the top of the function is also an optimisation, most likely done during the register allocation pass.
This was only possible because all functions take the same value in the same register. Otherwise, there would probably have been also a `mov` before each of the calls.
gcc unfortunately cannot do this (and also not the related optimization of using word/byte offsets for the jump table, which LLVM can do) because it only has approximate estimation of code size. If it got the code size wrong it would be a mis-compilation. This is related to it not using an integrated assembler.
I remember having implemented similar things in JavaScript (by generating code), for immutable Maps and Sets - no significant performance gain in modern engines. For regular expressions of complex unicode ranges, the speedup was around 2x!
In order to peak into hardware speculation/BTB, having a look atswitch lowering seems a good way. Big switches will go into serious convolution to please that hardware since by definition this is a source of a lot of branch mispredicitions.
There is surprisingly little about the proper feeding of advanced hardware in that code, two or three constants in cost calculations of which one or two are hardware-dependent.
It is still possible that a later pass arranges jump targets to avoid false sharing of their entries (addresses) in prediction mechanisms and such.
For example, my bug report that `+= 2` is broken has not been acknowledged (read?) by anyone for 3 years:
https://github.com/llvm/llvm-project/issues/70687Ahh, there it is! This issue isn't being tended to because it will resolve itself with the imminent death of OpenMP.
>The LLVM patch was rejected a decade ago
And what was the reasoning?
It could not because that would ensure that 7 values for each case would be handled by function calls instead of the default case - 0/8 is 0, but 3/8 is also 0
A rough benchmark of a version of that against the original shows it to be close to twice as fast for unpredictable input with GCC (4.5 ns vs 2.72 ns) on my old machine. With predictable input where x values often are the same the original version is quicker
... unless the size of immediate parameters would have affected code size. In the example code however, all constants fit in an `imm8` parameter to `cmp`.
Each `jmp` to f0..fX is an optimisation of a `call` and a `ret`, and probably done in a separate pass after the switch-case code has already been expanded.
The `mov edi, esi` at the top of the function is also an optimisation, most likely done during the register allocation pass. This was only possible because all functions take the same value in the same register. Otherwise, there would probably have been also a `mov` before each of the calls.
It is still possible that a later pass arranges jump targets to avoid false sharing of their entries (addresses) in prediction mechanisms and such.
Obvsiously engaging in pleasing as much as it can speculative/BTB hardware (to the cost of memory tables!)