Auto merge of #42556 - scottmcm:ctz-nz, r=BurntSushi
Get LLVM to stop generating dead assembly in next_power_of_two It turns out that LLVM can turn `@llvm.ctlz.i64(_, true)` into `@llvm.ctlz.i64(_, false)` ([`ctlz`](http://llvm.org/docs/LangRef.html#llvm-ctlz-intrinsic)) where valuable, but never does the opposite. That leads to some silly assembly getting generated in certain cases. A contrived-but-clear example https://is.gd/VAIKuC: ```rust fn foo(x:u64) -> u32 { if x == 0 { return !0; } x.leading_zeros() } ``` Generates ```asm testq %rdi, %rdi je .LBB0_1 je .LBB0_3 ; <-- wha? bsrq %rdi, %rax xorq $63, %rax retq .LBB0_1: movl $-1, %eax retq .LBB0_3: movl $64, %eax ; <-- dead retq ``` I noticed this in `next_power_of_two`, which without this PR generates the following: ```asm cmpq $2, %rcx jae .LBB1_2 movl $1, %eax retq .LBB1_2: decq %rcx je .LBB1_3 bsrq %rcx, %rcx xorq $63, %rcx jmp .LBB1_5 .LBB1_3: movl $64, %ecx ; <-- dead .LBB1_5: movq $-1, %rax shrq %cl, %rax incq %rax retq ``` And with this PR becomes ```asm cmpq $2, %rcx jae .LBB0_2 movl $1, %eax retq .LBB0_2: decq %rcx bsrq %rcx, %rcx xorl $63, %ecx movq $-1, %rax shrq %cl, %rax incq %rax retq ```
Showing
想要评论请 注册 或 登录