[llvm] [InstCombine] Fold copysign(floor(fabs(X)), X) to trunc(X) (PR #200836)

[Aayush Shrivastava via llvm-commits]

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@@ -3116,6 +3116,16 @@ Instruction *InstCombinerImpl::visitCallInst(CallInst &CI) {
     if (match(Mag, m_FAbs(m_Value(X))) || match(Mag, m_FNeg(m_Value(X))))
       return replaceOperand(*II, 0, X);
 
+    // copysign(floor(fabs(X)), X) --> trunc(X)
+    // Requires nnan: for NaN X the source returns a NaN with X's sign bit
+    // preserved (via copysign), but trunc(X) may return any NaN, making the
+    // target more undefined.
+    if (II->hasNoNaNs() &&
+        match(Mag, m_Intrinsic<Intrinsic::floor>(m_FAbs(m_Specific(Sign))))) {
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iamaayushrivastava wrote:

Thanks for the suggestion! Updated the fold to `copysign(trunc(X), X`) by replacing only the magnitude operand. This keeps the copysign alive so it's implicit fabs on the magnitude handles NaN correctly without requiring `nnan`.

https://github.com/llvm/llvm-project/pull/200836