Duplicate unnecessary null checks

[BruceForstall]

Function System.CSharpHelpers:.cctor() starts with a call to new (this is in R2R code):

call     [CORINFO_HELP_READYTORUN_NEW]
mov      rsi, rax
mov      rcx, rsi
call     [System.Collections.Generic.HashSet`1[__Canon][System.__Canon]:.ctor():this]

and then has a long sequence of:

mov      rdx, qword ptr [(reloc)]
mov      rdx, gword ptr [rdx]
mov      rcx, rsi
lea      r11, [(reloc)]
cmp      dword ptr [rcx], ecx
call     [System.Collections.Generic.HashSet`1[__Canon][System.__Canon]:Add(System.__Canon):bool:this]

It seems like only one cmp dword ptr [rcx], ecx is required; the rest are all doing the same work.

In fact, since rcx is the result of the call to new, in this function, it seems like the JIT should know it is non-null, and shouldn't need to do any null checks.

category:cq
theme:null-checks
skill-level:expert
cost:medium