[RISCV] Propagate SDNode flags when combining (fmul (fneg X), ...) (#169460)

In #157388, we turned `(fmul (fneg X), Y)` into `(fneg (fmul X, Y))`.
However, we forgot to propagate SDNode flags, specifically fast math
flags, from the original FMUL to the new one. This hinders some of the
subsequent (FMA) DAG combiner patterns that relied on the contraction
flag and as a consequence, missed some of the opportunities to generate
negation FMA instructions like `fnmadd`.

This patch fixes this issue by propagating the flags.

---------

Co-authored-by: Craig Topper <[email protected]>

Author: mshockwave

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -20822,7 +20822,8 @@ SDValue RISCVTargetLowering::PerformDAGCombine(SDNode *N,
     // Undo this and sink the fneg so we match more fmsub/fnmadd patterns.
     if (sd_match(N, m_FMul(m_Value(X), m_OneUse(m_FNeg(m_Value(Y))))))
       return DAG.getNode(ISD::FNEG, DL, VT,
-                         DAG.getNode(ISD::FMUL, DL, VT, X, Y));
+                         DAG.getNode(ISD::FMUL, DL, VT, X, Y, N->getFlags()),
+                         N->getFlags());
 
     // fmul X, (copysign 1.0, Y) -> fsgnjx X, Y
     SDValue N0 = N->getOperand(0);

llvm/test/CodeGen/RISCV/fma-combine.ll

@@ -0,0 +1,56 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 6
+; RUN: llc -mtriple=riscv64 -mattr=+d,+m < %s | FileCheck %s
+
+; What the original PR (#169460) tried to solve can only be revealed when a specific
+; set of FMA DAG combiner patterns were skipped due to hitting some recursion limits.
+; And this test is written in a way to hit that limit.
+
+define double @fnmadd_non_trivial(ptr %p0, ptr %p1, ptr %dst, double %mul425) {
+; CHECK-LABEL: fnmadd_non_trivial:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    li a3, -2047
+; CHECK-NEXT:    slli a3, a3, 51
+; CHECK-NEXT:    fmv.d.x fa5, a3
+; CHECK-NEXT:    lui a3, 2049
+; CHECK-NEXT:    slli a3, a3, 39
+; CHECK-NEXT:    fmv.d.x fa4, a3
+; CHECK-NEXT:    lui a3, 8201
+; CHECK-NEXT:    slli a3, a3, 37
+; CHECK-NEXT:    fmv.d.x fa3, a3
+; CHECK-NEXT:    li a3, 1023
+; CHECK-NEXT:    fmv.d.x fa2, zero
+; CHECK-NEXT:    slli a3, a3, 52
+; CHECK-NEXT:    fsub.d fa1, fa2, fa0
+; CHECK-NEXT:    fmadd.d fa1, fa1, fa3, fa4
+; CHECK-NEXT:    fmadd.d fa4, fa0, fa3, fa4
+; CHECK-NEXT:    fmv.d.x fa3, a3
+; CHECK-NEXT:    lui a3, %hi(.LCPI0_0)
+; CHECK-NEXT:    ld a3, %lo(.LCPI0_0)(a3)
+; CHECK-NEXT:    fmul.d fa5, fa0, fa5
+; CHECK-NEXT:    fnmadd.d fa4, fa4, fa2, fa3
+; CHECK-NEXT:    fnmadd.d fa3, fa1, fa2, fa3
+; CHECK-NEXT:    sd a3, 0(a2)
+; CHECK-NEXT:    fsd fa5, 0(a0)
+; CHECK-NEXT:    fnmadd.d fa5, fa4, fa2, fa0
+; CHECK-NEXT:    fnmadd.d fa0, fa0, fa2, fa3
+; CHECK-NEXT:    fsd fa5, 0(a1)
+; CHECK-NEXT:    ret
+  store double 0x3FEE666666666666, ptr %dst, align 8
+  %mul413 = fmul double %mul425, -3.000000e+00
+  store double %mul413, ptr %p0, align 8
+  %mul428 = fmul contract double %mul425, 4.500000e+00
+  %add429 = fadd nsz contract double %mul428, 3.000000e+00
+  %mul430 = fmul contract double %add429, 0.000000e+00
+  %sub432 = fadd nsz contract double %mul430, 1.000000e+00
+  %mul433 = fmul contract double %sub432, 0.000000e+00
+  %1 = fsub nsz contract double %mul433, %mul425
+  store double %1, ptr %p1, align 8
+  %mul441 = fmul contract double %mul425, 0.000000e+00
+  %add443 = fsub double 0.000000e+00, %mul425
+  %mul446 = fmul contract double %add443, 4.500000e+00
+  %add447 = fadd nsz contract double %mul446, 3.000000e+00
+  %mul448 = fmul contract double %add447, 0.000000e+00
+  %sub450 = fadd nsz contract double %mul448, 1.000000e+00
+  %2 = fsub nsz contract double %sub450, %mul441
+  ret double %2
+}