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Alyssa Rosenzweig
bd4464bd5e
Instruction count CI has transformed the way we work on FEX… I love the system and want to make it better. there’s one part of instruction count CI that isn’t so lovable: the problematic “optimal” flag on instructions. There are several issues with this flag, both philosophical and practical. – it is tedious to update the optimal flag when making an implementation optimal. The effect of that is discouraging people from making instructions, optimal, or encouraging people to fail to update the flag, and dilute the value of it. Either way, since we care far more about optimal implementations, then we do about updating the flag, clearly we should prioritize the implementation and not the flag. This issue was not obvious at the outset, when instruction count, CI was introduced, and still quite small. The problem magnified when we started duplicating instructions in bulk for different combinations of CPU features (flagm, AFP, etc.) that intern multiplies the manual work required to update the flags by the corresponding constant factor. if it comes down to a choice between removing this extra coverage and removing the flag, I think we all agree that removing the flag is the lesser evil. – The definition of “optimal” is fundamentally problematic. I have often improved the instruction count of an instruction that was already “optimal”. This is all kinds of silly, and calls into question whether there’s any value whatsoever in the existing classifications of the flag. Furthermore, it is often unknowable, whether an implementation really is optimal. Is it possible to implement BZHI (with flag calculations) in fewer than eight instructions? We don’t know, and it’s silly to pretend that we do. – as a consequence of the problematic definitions , there are so many errors in both directions that I don’t think there’s much value in preserving the existing classification at the expense of +progress. Being able to say “32% of instructions are translated optimally” is neat, but it really doesn’t tell us anything whatsoever when you dig a little deeper. So, as the flag is misleading at best and perhaps harmful at worst, let’s remove it and make the instruction count CI, more useful overall. let’s let the expected count and the assembly speak for themselves, and cut away the chaff. if we want a meaningless number to report to management, we can instead calculate the average blowup factor ;-) Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
115 lines
2.4 KiB
JSON
115 lines
2.4 KiB
JSON
{
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"Features": {
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"Bitness": 64,
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"EnabledHostFeatures": [
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"AFP"
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],
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"DisabledHostFeatures": [
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"SVE128",
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"SVE256"
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]
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},
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"Instructions": {
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"roundss xmm0, xmm1, 00000000b": {
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"ExpectedInstructionCount": 1,
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"Comment": [
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"Nearest rounding",
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"0x66 0x0f 0x3a 0x0a"
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],
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"ExpectedArm64ASM": [
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"frintn s16, s17"
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]
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},
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"roundss xmm0, xmm1, 00000001b": {
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"ExpectedInstructionCount": 1,
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"Comment": [
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"-inf rounding",
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"0x66 0x0f 0x3a 0x0a"
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],
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"ExpectedArm64ASM": [
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"frintm s16, s17"
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]
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},
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"roundss xmm0, xmm1, 00000010b": {
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"ExpectedInstructionCount": 1,
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"Comment": [
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"+inf rounding",
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"0x66 0x0f 0x3a 0x0a"
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],
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"ExpectedArm64ASM": [
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"frintp s16, s17"
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]
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},
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"roundss xmm0, xmm1, 00000011b": {
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"ExpectedInstructionCount": 1,
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"Comment": [
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"truncate rounding",
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"0x66 0x0f 0x3a 0x0a"
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],
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"ExpectedArm64ASM": [
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"frintz s16, s17"
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]
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},
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"roundss xmm0, xmm1, 00000100b": {
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"ExpectedInstructionCount": 1,
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"Comment": [
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"host rounding mode rounding",
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"0x66 0x0f 0x3a 0x0a"
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],
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"ExpectedArm64ASM": [
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"frinti s16, s17"
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]
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},
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"roundsd xmm0, xmm1, 00000000b": {
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"ExpectedInstructionCount": 1,
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"Comment": [
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"Nearest rounding",
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"0x66 0x0f 0x3a 0x0b"
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],
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"ExpectedArm64ASM": [
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"frintn d16, d17"
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]
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},
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"roundsd xmm0, xmm1, 00000001b": {
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"ExpectedInstructionCount": 1,
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"Comment": [
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"-inf rounding",
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"0x66 0x0f 0x3a 0x0b"
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],
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"ExpectedArm64ASM": [
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"frintm d16, d17"
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]
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},
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"roundsd xmm0, xmm1, 00000010b": {
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"ExpectedInstructionCount": 1,
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"Comment": [
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"+inf rounding",
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"0x66 0x0f 0x3a 0x0b"
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],
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"ExpectedArm64ASM": [
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"frintp d16, d17"
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]
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},
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"roundsd xmm0, xmm1, 00000011b": {
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"ExpectedInstructionCount": 1,
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"Comment": [
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"truncate rounding",
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"0x66 0x0f 0x3a 0x0b"
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],
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"ExpectedArm64ASM": [
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"frintz d16, d17"
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]
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},
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"roundsd xmm0, xmm1, 00000100b": {
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"ExpectedInstructionCount": 1,
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"Comment": [
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"host rounding mode rounding",
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"0x66 0x0f 0x3a 0x0b"
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],
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"ExpectedArm64ASM": [
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"frinti d16, d17"
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]
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}
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}
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}
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