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Do value-extensions at ABI boundaries only when ABI requires it.

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There has been some confusion over the meaning of the "sign-extend"
(`sext`) and "zero-extend" (`uext`) attributes on parameters and return
values in signatures. According to the three implemented backends, these
attributes indicate that a value narrower than a full register should
always be extended in the way specified. However, they are much more
useful if they mean "extend in this way if the ABI requires extending":
only the ABI backend knows whether or not a particular ABI (e.g., x64
SysV vs. x64 Baldrdash) requires extensions, while only the frontend
(CLIF generator) knows whether or not a value is signed, so the two have
to work in concert.

This is the result of some very helpful discussion in #2354 (thanks to
@uweigand for raising the issue and @bjorn3 for helping to reason about
it).

This change respects the extension attributes in the above way, rather
than unconditionally extending, to avoid potential performance
degradation as we introduce more extension attributes on signatures.
This commit is contained in:
Chris Fallin
2020-11-04 11:45:44 -08:00
parent 285edeec3e
commit a2bbb198de
7 changed files with 98 additions and 47 deletions
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15
cranelift/codegen/src/ir/extfunc.rs
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@@ -256,8 +256,19 @@ impl fmt::Display for AbiParam {
/// Function argument extension options.
///
/// On some architectures, small integer function arguments are extended to the width of a
/// general-purpose register.
/// On some architectures, small integer function arguments and/or return values are extended to
/// the width of a general-purpose register.
///
/// This attribute specifies how an argument or return value should be extended *if the platform
/// and ABI require it*. Because the frontend (CLIF generator) does not know anything about the
/// particulars of the target's ABI, and the CLIF should be platform-independent, these attributes
/// specify *how* to extend (according to the signedness of the original program) rather than
/// *whether* to extend.
///
/// For example, on x86-64, the SystemV ABI does not require extensions of narrow values, so these
/// `ArgumentExtension` attributes are ignored; but in the Baldrdash (SpiderMonkey) ABI on the same
/// platform, all narrow values *are* extended, so these attributes may lead to extra
/// zero/sign-extend instructions in the generated machine code.
#[derive(Copy, Clone, PartialEq, Eq, Debug, Hash)]
#[cfg_attr(feature = "enable-serde", derive(Serialize, Deserialize))]
pub enum ArgumentExtension {