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Fix SInt literals to reject too small of widths #4786

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Merged
jackkoenig merged 2 commits into from
Mar 12, 2025
Merged

Fix SInt literals to reject too small of widths #4786

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fdf89d1
Fix SInt literals to reject too small of widths
jackkoenig Mar 11, 2025
39c7f75
Update core/src/main/scala/chisel3/internal/firrtl/IR.scala
jackkoenig Mar 11, 2025
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4 changes: 3 additions & 1 deletion core/src/main/scala/chisel3/internal/firrtl/IR.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -171,7 +171,9 @@ private[chisel3] object ir {
val unsigned = if (n < 0) (BigInt(1) << width.get) + n else n
s"asSInt(${ULit(unsigned, width).name})"
}
def minWidth: Int = (if (w.known) 0 else 1) + n.bitLength

// Special case for 0 which can be specified to zero-width (but defaults to 1 bit).
def minWidth: Int = if (n == 0 && w.known) 0 else 1 + n.bitLength

def cloneWithWidth(newWidth: Width): this.type = {
SLit(n, newWidth).asInstanceOf[this.type]
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14 changes: 14 additions & 0 deletions src/test/scala-2/chiselTests/SIntOps.scala
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Expand Up @@ -284,4 +284,18 @@ class SIntOpsSpec extends AnyPropSpec with Matchers with ShiftRightWidthBehavior
blit.x.litOption should be(Some(2))
blit.y.litOption should be(Some(9))
}

property("SInt literals with too small of a width should be rejected") {
// Sanity checks.
0.S.getWidth should be(1)
0.S(0.W).getWidth should be(0)
-1.S.getWidth should be(1)
1.S.getWidth should be(2)
// The real check.
-2.S.getWidth should be(2)
an[IllegalArgumentException] shouldBe thrownBy(-2.S(1.W))
0xde.S.getWidth should be(9)
an[IllegalArgumentException] shouldBe thrownBy(0xde.S(8.W))
an[IllegalArgumentException] shouldBe thrownBy(0xde.S(4.W))
}
Comment on lines +288 to +300
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@seldridge seldridge Mar 11, 2025

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These tests look right! I haven't thought through the other ramifications of this change, other than that the existing tests in the repo all pass.

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@jackkoenig jackkoenig Mar 11, 2025

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Yeah I'm confident it's correct. Note that even before this change 0xde.S.getWidth == 9, it's just that we were accepting 1-bit less than the correct answer if the user specified it which would then coincidentally do the right thing in most circumstances due to the FIRRTL we emitted actually having the correct width semantics. Thus things would work as long as you didn't introspect on the width and do things with it.

}