[Float2Int] Fix pessimization in the MinBW calculation. #86051
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The MinBW was being calculated using the significant bits of the upper and lower bounds. The upper bound is 1 past the last value in the range so I don't think it should be included. Instead get the min and max signed values from the range and use the significant bits of those. I'm still not sure if the +1 is needed after the std::max. I haven't investigated if its possible to add a test for this. We only look at whether the MinBW is > 32 right now. So the ranges need to be carefully crafted to see a difference.
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@llvm/pr-subscribers-llvm-transforms Author: Craig Topper (topperc) ChangesThe MinBW was being calculated using the significant bits of the upper and lower bounds. The upper bound is 1 past the last value in the range so I don't think it should be included. Instead get the min and max signed values from the range and use the significant bits of those. I'm still not sure if the +1 is needed after the std::max. I haven't investigated if its possible to add a test for this. We only look at whether the MinBW is > 32 right now. So the ranges need to be carefully crafted to see a difference. CC: @AtariDreams Full diff: https://github.com/llvm/llvm-project/pull/86051.diff 1 Files Affected:
diff --git a/llvm/lib/Transforms/Scalar/Float2Int.cpp b/llvm/lib/Transforms/Scalar/Float2Int.cpp
index ccca8bcc1a56ac..47042d96b46571 100644
--- a/llvm/lib/Transforms/Scalar/Float2Int.cpp
+++ b/llvm/lib/Transforms/Scalar/Float2Int.cpp
@@ -359,8 +359,8 @@ bool Float2IntPass::validateAndTransform() {
// The number of bits required is the maximum of the upper and
// lower limits, plus one so it can be signed.
- unsigned MinBW = std::max(R.getLower().getSignificantBits(),
- R.getUpper().getSignificantBits()) +
+ unsigned MinBW = std::max(R.getSignedMin().getSignificantBits(),
+ R.getSignedMax().getSignificantBits()) +
1;
LLVM_DEBUG(dbgs() << "F2I: MinBitwidth=" << MinBW << ", R: " << R << "\n");
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nikic
reviewed
Mar 21, 2024
| unsigned MinBW = std::max(R.getLower().getSignificantBits(), | ||
| R.getUpper().getSignificantBits()) + | ||
| unsigned MinBW = std::max(R.getSignedMin().getSignificantBits(), | ||
| R.getSignedMax().getSignificantBits()) + |
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Use R.getMinSignedBits() instead?
nikic
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Mar 21, 2024
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Mar 23, 2024
The MinBW was being calculated using the significant bits of the upper and lower bounds. The upper bound is 1 past the last value in the range so I don't think it should be included. Instead use ConstantRange::getMinSignedBits. I'm still not sure if the +1 is needed after the getMinSignedBits call.
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The MinBW was being calculated using the significant bits of the upper and lower bounds. The upper bound is 1 past the last value in the range so I don't think it should be included. Instead get the min and max signed values from the range and use the significant bits of those.
I'm still not sure if the +1 is needed after the std::max.
I haven't investigated if its possible to add a test for this. We only look at whether the MinBW is > 32 right now. So the ranges need to be carefully crafted to see a difference.
CC: @AtariDreams