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[codegen] add documentation for overflow

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Add documentation to the icmp instruction text for both signed and
unsigned overflow, making it very clear why unsigned overflow is
complicated and where to find it.
This commit is contained in:
Ujjwal Sharma
2019-09-26 03:38:38 +05:30
committed by Dan Gohman
parent e45ef24d3b
commit 9f3c5b967e
1 changed files with 15 additions and 12 deletions
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27
cranelift/codegen/meta/src/shared/instructions.rs
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@@ -1586,8 +1586,6 @@ pub(crate) fn define(
let x = &operand("x", Int); let x = &operand("x", Int);
let y = &operand("y", Int); let y = &operand("y", Int);
// TODO(ryzokuken): Add documentation for unsigned overflow.
// TODO(ryzokuken): Add documentation for signed overflow.
ig.push( ig.push(
Inst::new( Inst::new(
"icmp", "icmp",
@@ -1597,16 +1595,21 @@ pub(crate) fn define(
The condition code determines if the operands are interpreted as signed The condition code determines if the operands are interpreted as signed
or unsigned integers. or unsigned integers.
====== ======== ========= | Signed | Unsigned | Condition |
Signed Unsigned Condition |--------|----------|-----------------------|
====== ======== ========= | eq | eq | Equal |
eq eq Equal | ne | ne | Not equal |
ne ne Not equal | slt | ult | Less than |
slt ult Less than | sge | uge | Greater than or equal |
sge uge Greater than or equal | sgt | ugt | Greater than |
sgt ugt Greater than | sle | ule | Less than or equal |
sle ule Less than or equal | of | * | Overflow |
====== ======== ========= | nof | * | No Overflow |
\* The unsigned version of overflow conditions have ISA-specific
semantics and thus have been kept as methods on the TargetIsa trait as
[unsigned_add_overflow_condition][isa::TargetIsa::unsigned_add_overflow_condition] and
[unsigned_sub_overflow_condition][isa::TargetIsa::unsigned_sub_overflow_condition].
When this instruction compares integer vectors, it returns a boolean When this instruction compares integer vectors, it returns a boolean
vector of lane-wise comparisons. vector of lane-wise comparisons.