[RyuJIT] Div then Mod (or viceversa) executes idiv twice instead of using the value in the register #5213
Description
Found this trying to optimize a very low level memory addressing algorithm. Suddenly I noticed that we were doing the idiv twice.
Repro on rc2-16551:
public class Program
{
public long Dividend;
[MethodImpl(MethodImplOptions.NoInlining)]
public long Run(long parameter)
{
long div = parameter / Dividend;
long mod = parameter % Dividend;
return div + mod;
}
public static void Main(string[] args)
{
Random r = new Random();
var p = new Program();
p.Dividend = 10;
long value = p.Run(203);
Console.WriteLine($"{value}");
Console.ReadLine();
}
}will output (in x64 but will also happen in x86):
00007FF8E8EC3C83 mov rcx,qword ptr [rcx+8]
00007FF8E8EC3C87 mov rax,r8 // 2 instructions to prepare for idiv.
00007FF8E8EC3C8A cqo
00007FF8E8EC3C8C idiv rax,rcx // parameter / Dividend
00007FF8E8EC3C8F mov r9,rax // r9:= divResult
00007FF8E8EC3C92 mov rax,r8 // 2 instructions to prepare for idiv.
00007FF8E8EC3C95 cqo
00007FF8E8EC3C97 idiv rax,rcx // parameter / Dividend
00007FF8E8EC3C9A lea rax,[r9+rdx] // rax := divResult + modulusthis could be written as:
00007FF8E8EC3C83 mov rcx,qword ptr [rcx+8]
00007FF8E8EC3C87 mov rax,r8 // 2 instructions to prepare for idiv.
00007FF8E8EC3C8A cqo
00007FF8E8EC3C8C idiv rax,rcx // parameter / Dividend
00007FF8E8EC3C92 add rax,rdx // rax := divResult + modulusThis is pretty common in addressing modes or accessing matrix locations, etc. Everywhere you have a row and a column, you will have such an structure.
category:cq
theme:div-mod-rem
skill-level:expert
cost:large
impact:medium