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Clean up handling of NOPs in the x64 backend.

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1. Restricts max nop size to 15 instead of 16.
2. Fixes an edge case where gen_nop() would return a zero sized intruction on multiples of 16.
3. Clarifies the documentation of the gen_nop interface to state that returning zero is allowed when preferred_size is zero.
This commit is contained in:
Kasey Carrothers
2021-01-27 22:45:35 -08:00
parent 7f840870c7
commit f76a9d436e
2 changed files with 4 additions and 4 deletions
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6
cranelift/codegen/src/isa/x64/inst/mod.rs
View File

@@ -566,7 +566,7 @@ impl Inst {
impl Inst {
pub(crate) fn nop(len: u8) -> Self {
debug_assert!(len <= 16);
debug_assert!(len <= 15);
Self::Nop { len }
}
@@ -2594,11 +2594,11 @@ impl MachInst for Inst {
}
fn gen_zero_len_nop() -> Inst {
Inst::Nop { len: 0 }
Inst::nop(0)
}
fn gen_nop(preferred_size: usize) -> Inst {
Inst::nop((preferred_size % 16) as u8)
Inst::nop(std::cmp::min(preferred_size, 15) as u8)
}
fn maybe_direct_reload(&self, _reg: VirtualReg, _slot: SpillSlot) -> Option<Inst> {

2
cranelift/codegen/src/machinst/mod.rs
View File

@@ -178,7 +178,7 @@ pub trait MachInst: Clone + Debug {
/// request a NOP of that size, or as close to it as possible. The machine
/// backend may return a NOP whose binary encoding is smaller than the
/// preferred size, but must not return a NOP that is larger. However,
/// the instruction must have a nonzero size.
/// the instruction must have a nonzero size if preferred_size is nonzero.
fn gen_nop(preferred_size: usize) -> Self;
/// Get the register universe for this backend.