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Tweak memcpy optimisation #2003

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Merged
Fidget-Spinner merged 3 commits into from
Dec 28, 2025
Merged

Tweak memcpy optimisation #2003

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c6cb8f9
Homogenise comment widths.
ltratt Dec 28, 2025
e3ecdb9
Shorten to equivalent.
ltratt Dec 28, 2025
8a9a8d5
LLVM's `memcpy` has slightly different outcomes compared to libc.
ltratt Dec 28, 2025
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52 changes: 34 additions & 18 deletions ykrt/src/compile/j2/opt/strength_fold.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -126,9 +126,8 @@ fn opt_and(opt: &mut PassOpt, mut inst: And) -> OptOutcome {
return OptOutcome::Equiv(rhs);
}
if rhs_c == ArbBitInt::all_bits_set(rhs_c.bitw()) {
// Reduce `x & y` to `x` if `y` is a constant that has all
// the necessary bits set for this integer type. For an i1, for
// example, `x & 1` can be replaced with `x`.
// Reduce `x & y` to `x` if `y` is a constant that has all the necessary bits set
// for this integer type. For an i1, for example, `x & 1` can be replaced with `x`.
return OptOutcome::Equiv(lhs);
}
}
Expand Down Expand Up @@ -358,8 +357,8 @@ fn opt_guard(opt: &mut PassOpt, mut inst @ Guard { expect, cond, .. }: Guard) ->

let cond = opt.equiv_iidx(cond);
if let Inst::Const(_) = opt.inst(cond) {
// A guard that references a constant is, by definition, not needed and
// doesn't affect future analyses.
// A guard that references a constant is, by definition, not needed and doesn't affect
// future analyses.
return OptOutcome::NotNeeded;
}

Expand Down Expand Up @@ -494,20 +493,25 @@ fn opt_memcpy(opt: &mut PassOpt, mut inst: MemCpy) -> OptOutcome {
len,
volatile: _,
} = inst;
match (opt.as_constkind(dst), opt.as_constkind(src)) {
(Some(ConstKind::Ptr(lhs_c)), Some(ConstKind::Ptr(rhs_c))) if lhs_c == rhs_c => {
// memcpy where lhs_ptr == rhs_ptr. Not needed.
// This is technically UB, but GCC 15 and Clang 21
// both choose to optimise away the memcpy.
// Thus, we shall do the same.
return OptOutcome::NotNeeded;
}
_ => (),

// LLVM's `memcpy` allows `dst` and `src` to point to the same memory, at which point `memcpy`
// is a no-op.
let equiv = if dst == src {
true
} else if let (Some(ConstKind::Ptr(lhs_c)), Some(ConstKind::Ptr(rhs_c))) =
(opt.as_constkind(dst), opt.as_constkind(src))
&& lhs_c == rhs_c
{
true
} else {
false
};
if equiv {
return OptOutcome::NotNeeded;
}

if let Some(ConstKind::Int(len_c)) = opt.as_constkind(len)
&& let Some(len_as_int) = len_c.to_zero_ext_u64()
&& len_as_int == 0
&& let Some(0) = len_c.to_zero_ext_u8()
{
// memcpy of zero bytes. Not needed.
return OptOutcome::NotNeeded;
Expand Down Expand Up @@ -641,8 +645,8 @@ fn opt_or(opt: &mut PassOpt, mut inst: Or) -> OptOutcome {
return OptOutcome::Equiv(lhs);
}
if rhs_c == ArbBitInt::all_bits_set(rhs_c.bitw()) {
// Reduce `x | y` to `y` if `y` is a constant that has all
// the necessary bits set for this integer type.
// Reduce `x | y` to `y` if `y` is a constant that has all the necessary bits set
// for this integer type.
return OptOutcome::Equiv(lhs);
}
}
Expand Down Expand Up @@ -2376,6 +2380,18 @@ mod test {
",
);

test_sf(
"
%0: ptr = arg [reg]
%1: i64 = 4
memcpy %0, %0, %1, false
",
"
%0: ptr = arg
%1: i64 = 4
",
);

// memcpy where src!=dst is not eliminated.
test_sf(
"
Expand Down