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Rollup merge of rust-lang#103287 - saethlin:faster-len-check, r=thomcc
Use a faster allocation size check in slice::from_raw_parts I've been perusing through the codegen changes that result from turning on the standard library debug assertions. The previous check in here uses saturating arithmetic, which in my experience sometimes makes LLVM just fail to optimize things around the saturating operation. Here is a demo of the codegen difference: https://godbolt.org/z/WMEqrjajW Before: ```asm example::len_check_old: mov rax, rdi mov ecx, 3 mul rcx setno cl test rax, rax setns al and al, cl ret example::len_check_old: mov rax, rdi mov ecx, 8 mul rcx setno cl test rax, rax setns al and al, cl ret ``` After: ```asm example::len_check_new: movabs rax, 3074457345618258603 cmp rdi, rax setb al ret example::len_check_new: shr rdi, 60 sete al ret ``` Running rustc-perf locally, this looks like up to a 4.5% improvement when `debug-assertions-std = true`. Thanks `@LegionMammal978` (I think that's you?) for turning my idea into a much cleaner implementation. r? `@thomcc`
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