[llvm] [LV] Handle FSub Partial Reductions (PR #191186)

[Benjamin Maxwell via llvm-commits]

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@@ -8792,17 +8793,28 @@ static SmallVector<Instruction *> preparePlanForEpilogueVectorLoop(
           // Partial sub-reductions always start at 0 and account for the
           // reduction start value in a final subtraction. Update it to use the
           // resume value from the main vector loop.
-          if (PhiR->getVFScaleFactor() > 1 &&
-              PhiR->getRecurrenceKind() == RecurKind::Sub) {
+          if ((PhiR->getVFScaleFactor() > 1 &&
+               PhiR->getRecurrenceKind() == RecurKind::Sub) ||
+              PhiR->getRecurrenceKind() == RecurKind::FSub) {
             auto *Sub = cast<VPInstruction>(RdxResult->getSingleUser());
-            assert(Sub->getOpcode() == Instruction::Sub && "Unexpected opcode");
+            assert((Sub->getOpcode() == Instruction::Sub ||
+                    Sub->getOpcode() == Instruction::FSub) &&
+                   "Unexpected opcode");
             assert(isa<VPIRValue>(Sub->getOperand(0)) &&
                    "Expected operand to match the original start value of the "
                    "reduction");
-            assert(VPlanPatternMatch::match(VPI->getOperand(0),
-                                            VPlanPatternMatch::m_ZeroInt()) &&
-                   "Expected start value for partial sub-reduction to start at "
-                   "zero");
+            // For integer sub-reductions, verify start value is zero.
+            // For FP sub-reductions, verify start value is negative zero.
+            auto StartValueIsIdentity = [&] {
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MacDue wrote:

I think this needs `[[maybe_unused]]` (or a `(void)StartValueIsIdentity;` after the assert) to prevent warnings about an unused variables in release builds.  


https://github.com/llvm/llvm-project/pull/191186