[llvm] [KnownBits] Add KnownBits::absdiff to compute the absolute difference of 2 unsigned values (PR #82354)
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Tue Feb 20 04:49:40 PST 2024
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-llvm-support
Author: Simon Pilgrim (RKSimon)
<details>
<summary>Changes</summary>
Equivalent to "umax(A, B) - umin(A, B)"
This is just an initial correctness implementation, hopefully we can make this optimal in the future.
---
Full diff: https://github.com/llvm/llvm-project/pull/82354.diff
3 Files Affected:
- (modified) llvm/include/llvm/Support/KnownBits.h (+3)
- (modified) llvm/lib/Support/KnownBits.cpp (+17)
- (modified) llvm/unittests/Support/KnownBitsTest.cpp (+8-1)
``````````diff
diff --git a/llvm/include/llvm/Support/KnownBits.h b/llvm/include/llvm/Support/KnownBits.h
index fb034e0b9e3baf..69c569b97ccae1 100644
--- a/llvm/include/llvm/Support/KnownBits.h
+++ b/llvm/include/llvm/Support/KnownBits.h
@@ -385,6 +385,9 @@ struct KnownBits {
/// Compute known bits for smin(LHS, RHS).
static KnownBits smin(const KnownBits &LHS, const KnownBits &RHS);
+ /// Compute known bits for absdiff(LHS, RHS).
+ static KnownBits absdiff(const KnownBits &LHS, const KnownBits &RHS);
+
/// Compute known bits for shl(LHS, RHS).
/// NOTE: RHS (shift amount) bitwidth doesn't need to be the same as LHS.
static KnownBits shl(const KnownBits &LHS, const KnownBits &RHS,
diff --git a/llvm/lib/Support/KnownBits.cpp b/llvm/lib/Support/KnownBits.cpp
index 770e4051ca3ffa..e0ecdd56711959 100644
--- a/llvm/lib/Support/KnownBits.cpp
+++ b/llvm/lib/Support/KnownBits.cpp
@@ -176,6 +176,23 @@ KnownBits KnownBits::smin(const KnownBits &LHS, const KnownBits &RHS) {
return Flip(umax(Flip(LHS), Flip(RHS)));
}
+KnownBits KnownBits::absdiff(const KnownBits &LHS, const KnownBits &RHS) {
+ // absdiff(LHS,RHS) = sub(umax(LHS,RHS), umin(LHS,RHS)).
+ KnownBits UMaxValue = umax(LHS, RHS);
+ KnownBits UMinValue = umin(LHS, RHS);
+ KnownBits KnownAbsDiff = computeForAddSub(false, false, UMaxValue, UMinValue);
+
+ // fallback - find the common bits between sub(LHS,RHS) and sub(RHS,LHS).
+ if (KnownAbsDiff.isUnknown()) {
+ KnownBits Diff0 = computeForAddSub(false, false, LHS, RHS);
+ KnownBits Diff1 = computeForAddSub(false, false, RHS, LHS);
+ KnownAbsDiff = Diff0.intersectWith(Diff1);
+ }
+
+ assert(!KnownAbsDiff.hasConflict() && "Bad Output");
+ return KnownAbsDiff;
+}
+
static unsigned getMaxShiftAmount(const APInt &MaxValue, unsigned BitWidth) {
if (isPowerOf2_32(BitWidth))
return MaxValue.extractBitsAsZExtValue(Log2_32(BitWidth), 0);
diff --git a/llvm/unittests/Support/KnownBitsTest.cpp b/llvm/unittests/Support/KnownBitsTest.cpp
index c0377d45c303a1..dfa3fe06a32647 100644
--- a/llvm/unittests/Support/KnownBitsTest.cpp
+++ b/llvm/unittests/Support/KnownBitsTest.cpp
@@ -281,7 +281,14 @@ TEST(KnownBitsTest, BinaryExhaustive) {
return KnownBits::smin(Known1, Known2);
},
[](const APInt &N1, const APInt &N2) { return APIntOps::smin(N1, N2); });
-
+ testBinaryOpExhaustive(
+ [](const KnownBits &Known1, const KnownBits &Known2) {
+ return KnownBits::absdiff(Known1, Known2);
+ },
+ [](const APInt &N1, const APInt &N2) {
+ return APIntOps::absdiff(N1, N2);
+ },
+ checkCorrectnessOnlyBinary);
testBinaryOpExhaustive(
[](const KnownBits &Known1, const KnownBits &Known2) {
return KnownBits::udiv(Known1, Known2);
``````````
</details>
https://github.com/llvm/llvm-project/pull/82354
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