Vectorizer: track non-fast FP instructions through phis when finding reductions.

When we traced through a phi node looking for floating-point reductions, we
forgot whether we'd ever seen an instruction without fast-math flags (that
would block vectorization). This propagates it through to the end.

llvm-svn: 271015

llvm/lib/Transforms/Utils/LoopUtils.cpp

@@ -430,7 +430,7 @@ RecurrenceDescriptor::isRecurrenceInstr(Instruction *I, RecurrenceKind Kind,
   default:
     return InstDesc(false, I);
   case Instruction::PHI:
-    return InstDesc(I, Prev.getMinMaxKind());
+    return InstDesc(I, Prev.getMinMaxKind(), Prev.getUnsafeAlgebraInst());
   case Instruction::Sub:
   case Instruction::Add:
     return InstDesc(Kind == RK_IntegerAdd, I);

llvm/test/Transforms/LoopVectorize/imprecise-through-phis.ll

@@ -0,0 +1,75 @@
+; RUN: opt -S -loop-vectorize -mtriple=x86_64-apple-darwin %s | FileCheck %s
+
+; Two mostly identical functions. The only difference is the presence of
+; fast-math flags on the second. The loop is a pretty simple reduction:
+
+; for (int i = 0; i < 32; ++i)
+;   if (arr[i] != 42)
+;     tot += arr[i];
+
+define double @sumIfScalar(double* nocapture readonly %arr) {
+; CHECK-LABEL: define double @sumIfScalar
+; CHECK-NOT: <2 x double>
+
+entry:
+  br label %loop
+
+loop:
+  %i = phi i32 [0, %entry], [%i.next, %next.iter]
+  %tot = phi double [0.0, %entry], [%tot.next, %next.iter]
+
+  %addr = getelementptr double, double* %arr, i32 %i
+  %nextval = load double, double* %addr
+
+  %tst = fcmp une double %nextval, 42.0
+  br i1 %tst, label %do.add, label %no.add
+
+do.add:
+  %tot.new = fadd double %tot, %nextval
+  br label %next.iter
+
+no.add:
+  br label %next.iter
+
+next.iter:
+  %tot.next = phi double [%tot, %no.add], [%tot.new, %do.add]
+  %i.next = add i32 %i, 1
+  %again = icmp ult i32 %i.next, 32
+  br i1 %again, label %loop, label %done
+
+done:
+  ret double %tot.next
+}
+
+define double @sumIfVector(double* nocapture readonly %arr) {
+; CHECK-LABEL: define double @sumIfVector
+; CHECK: <2 x double>
+entry:
+  br label %loop
+
+loop:
+  %i = phi i32 [0, %entry], [%i.next, %next.iter]
+  %tot = phi double [0.0, %entry], [%tot.next, %next.iter]
+
+  %addr = getelementptr double, double* %arr, i32 %i
+  %nextval = load double, double* %addr
+
+  %tst = fcmp fast une double %nextval, 42.0
+  br i1 %tst, label %do.add, label %no.add
+
+do.add:
+  %tot.new = fadd fast double %tot, %nextval
+  br label %next.iter
+
+no.add:
+  br label %next.iter
+
+next.iter:
+  %tot.next = phi double [%tot, %no.add], [%tot.new, %do.add]
+  %i.next = add i32 %i, 1
+  %again = icmp ult i32 %i.next, 32
+  br i1 %again, label %loop, label %done
+
+done:
+  ret double %tot.next
+}