[RISCV][Zba] Optimize mul with SH*ADD #68144
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Optimized multiplication by constants 23, 29, 35, 39, 43, 47, 49, 51, 53, 55, 57, 59, 61, 69, 75, 77, 83, 85, 87, 89, 91, 93, 95, 97, 99, 101, 103, 105, 107, 109, 111, 113, 117, 121, 123, 125, 135, 137, 145, 147, 149, 153, 155, 157, 161, 163, 165, 169, 171, 173, 181, 185, 189, 201, 203, 205, 217, 219, 225, 243, 293, 297, 301, 305, 325, 329, 333, 361, 365, 369, 405, 585, 593, 649, 657, 729
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Due to the new optimization with shXadds, the old tests expect a "naive mul", replaced it with a new constant that was not replaced by shXadds.
topperc
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Nov 24, 2023
| @@ -983,6 +983,1299 @@ define i64 @mul81(i64 %a) { | |||
| ret i64 %c | |||
| } | |||
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| define i64 @mul153(i64 %a) { | |||
| ; RV64I-LABEL: mul153: | |||
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llvm-mca doesn't think this is an improvement for sifive-x280 https://godbolt.org/z/6ahP11xrq
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Thanks for your feedback,
tests show that 3 shXadds is a little slower than mul, but for some reason GCC uses 3 shXadds - https://godbolt.org/z/oez5Pddde
And even 4 - https://godbolt.org/z/czK3Kd7bs
I'll try to find out why they do this and write later.
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This patch does the optimization of mul with a constant by different patterns of SH*ADD.
Optimized multiplication by constants 23, 29, 35, 39, 43, 47, 49, 51, 53, 55, 57, 59, 61, 69, 75, 77, 83, 85, 87, 89, 91, 93, 95, 97, 99, 101, 103, 105, 107, 109, 111, 113, 117, 121, 123, 125, 135, 137, 145, 147, 149, 153, 155, 157, 161, 163, 165, 169, 171, 173, 181, 185, 189, 201, 203, 205, 217, 219, 225, 243, 293, 297, 301, 305, 325, 329, 333, 361, 365, 369, 405, 585, 593, 649, 657, 729
Example of patterns:
shXadd a1 a0 a0
shYadd a0 a1 a1
shZadd a0 a1 a0
Mathematically, this is the same as multiplying by (2^x + 1)(2^z) + (2^x + 1)(2^y + 1). You can go through all X, Y, Z and find those constants that optimization cannot generate.