Get LLVM to stop generating dead assembly in next_power_of_two #42556
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r? @BurntSushi (rust_highfive has picked a reviewer for you, use r? to override) |
kennytm
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Jun 9, 2017
src/librustc_trans/intrinsic.rs
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| @@ -280,6 +280,11 @@ pub fn trans_intrinsic_call<'a, 'tcx>(bcx: &Builder<'a, 'tcx>, | |||
| let llfn = ccx.get_intrinsic(&format!("llvm.{}.i{}", name, width)); | |||
| bcx.call(llfn, &[llargs[0], y], None) | |||
| } | |||
| "ctlz_nonzero" | "cttz_nonzero" => { | |||
| let y = C_bool(bcx.ccx, true); | |||
| let llfn = ccx.get_intrinsic(&format!("llvm.{}.i{}", &name[..4], width)); | |||
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[00:03:22] tidy error: /checkout/src/librustc_trans/intrinsic.rs:285: line longer than 100 chars
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Yup; currently waiting on local double-check for the fix. Thanks for the targeted comment!
LLVM currently doesn't remove the "bypass if argument is zero" assembly inside branches where the value is known to be non-zero, pessimizing code that uses uN::leading_zeros
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LGTM. Thanks! @bors r+ |
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📌 Commit 6d86f0c has been approved by |
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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 ```
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☀️ Test successful - status-appveyor, status-travis |
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It turns out that LLVM can turn
@llvm.ctlz.i64(_, true)into@llvm.ctlz.i64(_, false)(ctlz) 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:
Generates
I noticed this in
next_power_of_two, which without this PR generates the following:And with this PR becomes