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Fix the carry semantics of _addcarry_u32 and _addcarry_u64

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The input carry is an 8-bit value that is interpreted as 1 when it is non-zero. The output carry is an 8-bit value that will be 0 or 1.

https://www.intel.com/content/www/us/en/docs/cpp-compiler/developer-guide-reference/2021-8/addcarry-u32-addcarry-u64.html
This commit is contained in:
Eduardo Sánchez Muñoz 2023-09-21 20:25:36 +02:00
parent 356039985e
commit 6ec63ed251
2 changed files with 21 additions and 8 deletions
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13
src/tools/miri/src/shims/x86/mod.rs
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@ -26,15 +26,17 @@ pub(super) trait EvalContextExt<'mir, 'tcx: 'mir>:
let unprefixed_name = link_name.as_str().strip_prefix("llvm.x86.").unwrap(); let unprefixed_name = link_name.as_str().strip_prefix("llvm.x86.").unwrap();
match unprefixed_name { match unprefixed_name {
// Used to implement the `_addcarry_u32` and `_addcarry_u64` functions. // Used to implement the `_addcarry_u32` and `_addcarry_u64` functions.
// Computes u8+uX+uX (uX is u32 or u64), returning tuple (u8,uX) comprising // Computes a + b with input and output carry. The input carry is an 8-bit
// the output carry and truncated sum. // value, which is interpreted as 1 if it is non-zero. The output carry is
// an 8-bit value that will be 0 or 1.
// https://www.intel.com/content/www/us/en/docs/cpp-compiler/developer-guide-reference/2021-8/addcarry-u32-addcarry-u64.html
"addcarry.32" | "addcarry.64" => { "addcarry.32" | "addcarry.64" => {
if unprefixed_name == "addcarry.64" && this.tcx.sess.target.arch != "x86_64" { if unprefixed_name == "addcarry.64" && this.tcx.sess.target.arch != "x86_64" {
return Ok(EmulateByNameResult::NotSupported); return Ok(EmulateByNameResult::NotSupported);
} }
let [c_in, a, b] = this.check_shim(abi, Abi::Unadjusted, link_name, args)?; let [c_in, a, b] = this.check_shim(abi, Abi::Unadjusted, link_name, args)?;
let c_in = this.read_scalar(c_in)?.to_u8()?; let c_in = this.read_scalar(c_in)?.to_u8()? != 0;
let a = this.read_immediate(a)?; let a = this.read_immediate(a)?;
let b = this.read_immediate(b)?; let b = this.read_immediate(b)?;
@ -44,10 +46,9 @@ pub(super) trait EvalContextExt<'mir, 'tcx: 'mir>:
&sum, &sum,
&ImmTy::from_uint(c_in, a.layout), &ImmTy::from_uint(c_in, a.layout),
)?; )?;
#[allow(clippy::arithmetic_side_effects)] // adding two bools into a u8 let c_out = overflow1 | overflow2;
let c_out = u8::from(overflow1) + u8::from(overflow2);
this.write_scalar(Scalar::from_u8(c_out), &this.project_field(dest, 0)?)?; this.write_scalar(Scalar::from_u8(c_out.into()), &this.project_field(dest, 0)?)?;
this.write_immediate(*sum, &this.project_field(dest, 1)?)?; this.write_immediate(*sum, &this.project_field(dest, 1)?)?;
} }

16
src/tools/miri/tests/pass/intrinsics-x86.rs
View File

@ -14,8 +14,14 @@ mod x86 {
} }
pub fn main() { pub fn main() {
assert_eq!(adc(0, 1, 1), (0, 2));
assert_eq!(adc(1, 1, 1), (0, 3)); assert_eq!(adc(1, 1, 1), (0, 3));
assert_eq!(adc(3, u32::MAX, u32::MAX), (2, 1)); assert_eq!(adc(2, 1, 1), (0, 3)); // any non-zero carry acts as 1!
assert_eq!(adc(u8::MAX, 1, 1), (0, 3));
assert_eq!(adc(0, u32::MAX, u32::MAX), (1, u32::MAX - 1));
assert_eq!(adc(1, u32::MAX, u32::MAX), (1, u32::MAX));
assert_eq!(adc(2, u32::MAX, u32::MAX), (1, u32::MAX));
assert_eq!(adc(u8::MAX, u32::MAX, u32::MAX), (1, u32::MAX));
} }
} }
@ -32,8 +38,14 @@ mod x86_64 {
} }
pub fn main() { pub fn main() {
assert_eq!(adc(0, 1, 1), (0, 2));
assert_eq!(adc(1, 1, 1), (0, 3)); assert_eq!(adc(1, 1, 1), (0, 3));
assert_eq!(adc(3, u64::MAX, u64::MAX), (2, 1)); assert_eq!(adc(2, 1, 1), (0, 3)); // any non-zero carry acts as 1!
assert_eq!(adc(u8::MAX, 1, 1), (0, 3));
assert_eq!(adc(0, u64::MAX, u64::MAX), (1, u64::MAX - 1));
assert_eq!(adc(1, u64::MAX, u64::MAX), (1, u64::MAX));
assert_eq!(adc(2, u64::MAX, u64::MAX), (1, u64::MAX));
assert_eq!(adc(u8::MAX, u64::MAX, u64::MAX), (1, u64::MAX));
} }
} }