Use unsigned arithmetic builtins for UInt(N) operations #4740
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We were mistakenly using the signed builtins, which produce the same
lowering for add/multiply right now, but don't for division and modulus.
When manually flipping SignedOverflowIsUB on, the signed version of add
gains the nsw (no signed wrap) flag, while the unsigned version
(correctly after this change) does not:
// CHECK:STDOUT: define i32 @_Cadd_i32.Main(i32 %a, i32 %b) !dbg !4 {
// CHECK:STDOUT: entry:
// CHECK:STDOUT: %int.sadd = add nsw i32 %a, %b, !dbg !7
// CHECK:STDOUT: ret i32 %int.sadd, !dbg !8
// CHECK:STDOUT: }
// CHECK:STDOUT:
// CHECK:STDOUT: define i32 @_Cadd_u32.Main(i32 %a, i32 %b) !dbg !9 {
// CHECK:STDOUT: entry:
// CHECK:STDOUT: %int.uadd = add i32 %a, %b, !dbg !10
// CHECK:STDOUT: ret i32 %int.uadd, !dbg !11
// CHECK:STDOUT: }
zygoloid
approved these changes
Dec 26, 2024
| } | ||
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| impl forall [N:! IntLiteral()] UInt(N) as Mul { | ||
| fn Op[self: Self](other: Self) -> Self = "int.smul"; | ||
| fn Op[self: Self](other: Self) -> Self = "int.umul"; | ||
| } | ||
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| impl forall [N:! IntLiteral()] UInt(N) as Negate { | ||
| fn Op[self: Self]() -> Self = "int.snegate"; |
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Suggested change
| fn Op[self: Self]() -> Self = "int.snegate"; | |
| fn Op[self: Self]() -> Self = "int.unegate"; |
snegate traps on signed overflow (eg, (-0x8000_0000 as i32) / -1 is an error); unegate does not. This would be visible in the overflow flags in lowering if we emitted them; for now I think it is only visible in constant evaluation.
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We were mistakenly using the signed builtins, which produce the same lowering for add/multiply right now, but don't for division and modulus.
When manually flipping SignedOverflowIsUB on, the signed version of add gains the nsw (no signed wrap) flag, while the unsigned version (correctly after this change) does not: