[DAGCombine] Respect the uses when combine FMA for a*b+/-c*d

If it is a*b-c*d, it could be also folded into fma(a, b, -c*d) or fma(-c, d, a*b).
This patch is trying to respect the uses of a*b and c*d to make the best choice.

Differential Revision: https://reviews.llvm.org/D75982

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -11983,13 +11983,24 @@ SDValue DAGCombiner::visitFSUBForFMACombine(SDNode *N) {
     return SDValue();
   };
 
-  // fold (fsub (fmul x, y), z) -> (fma x, y, (fneg z))
-  if (SDValue V = tryToFoldXYSubZ(N0, N1))
-    return V;
-
-  // fold (fsub x, (fmul y, z)) -> (fma (fneg y), z, x)
-  if (SDValue V = tryToFoldXSubYZ(N0, N1))
-    return V;
+  // If we have two choices trying to fold (fsub (fmul u, v), (fmul x, y)),
+  // prefer to fold the multiply with fewer uses.
+  if (isContractableFMUL(N0) && isContractableFMUL(N1) &&
+      (N0.getNode()->use_size() > N1.getNode()->use_size())) {
+    // fold (fsub (fmul a, b), (fmul c, d)) -> (fma (fneg c), d, (fmul a, b))
+    if (SDValue V = tryToFoldXSubYZ(N0, N1))
+      return V;
+    // fold (fsub (fmul a, b), (fmul c, d)) -> (fma a, b, (fneg (fmul c, d)))
+    if (SDValue V = tryToFoldXYSubZ(N0, N1))
+      return V;
+  } else {
+    // fold (fsub (fmul x, y), z) -> (fma x, y, (fneg z))
+    if (SDValue V = tryToFoldXYSubZ(N0, N1))
+      return V;
+    // fold (fsub x, (fmul y, z)) -> (fma (fneg y), z, x)
+    if (SDValue V = tryToFoldXSubYZ(N0, N1))
+      return V;
+  }
 
   // fold (fsub (fneg (fmul, x, y)), z) -> (fma (fneg x), y, (fneg z))
   if (N0.getOpcode() == ISD::FNEG && isContractableFMUL(N0.getOperand(0)) &&

llvm/test/CodeGen/PowerPC/fma-precision.ll

@@ -5,10 +5,10 @@
 define double @fsub1(double %a, double %b, double %c, double %d)  {
 ; CHECK-LABEL: fsub1:
 ; CHECK:       # %bb.0: # %entry
-; CHECK-NEXT:    xsmuldp 3, 4, 3
 ; CHECK-NEXT:    xsmuldp 0, 2, 1
-; CHECK-NEXT:    xsmsubadp 3, 2, 1
-; CHECK-NEXT:    xsmuldp 1, 0, 3
+; CHECK-NEXT:    fmr 1, 0
+; CHECK-NEXT:    xsnmsubadp 1, 4, 3
+; CHECK-NEXT:    xsmuldp 1, 0, 1
 ; CHECK-NEXT:    blr
 entry:
   %mul = fmul fast double %b, %a
@@ -101,13 +101,12 @@ entry:
 define double @fma_multi_uses1(double %a, double %b, double %c, double %d, double* %p1, double* %p2, double* %p3) {
 ; CHECK-LABEL: fma_multi_uses1:
 ; CHECK:       # %bb.0:
-; CHECK-NEXT:    xsmuldp 5, 1, 2
+; CHECK-NEXT:    xsmuldp 1, 1, 2
 ; CHECK-NEXT:    xsmuldp 0, 3, 4
-; CHECK-NEXT:    stfd 5, 0(7)
-; CHECK-NEXT:    stfd 5, 0(8)
+; CHECK-NEXT:    stfd 1, 0(7)
+; CHECK-NEXT:    stfd 1, 0(8)
+; CHECK-NEXT:    xsnmsubadp 1, 3, 4
 ; CHECK-NEXT:    stfd 0, 0(9)
-; CHECK-NEXT:    xsmsubadp 0, 1, 2
-; CHECK-NEXT:    fmr 1, 0
 ; CHECK-NEXT:    blr
   %ab = fmul fast double %a, %b
   %cd = fmul fast double %c, %d
@@ -142,16 +141,14 @@ define double @fma_multi_uses3(double %a, double %b, double %c, double %d, doubl
 ; CHECK-LABEL: fma_multi_uses3:
 ; CHECK:       # %bb.0:
 ; CHECK-NEXT:    xsmuldp 0, 1, 2
-; CHECK-NEXT:    xsmuldp 3, 3, 4
+; CHECK-NEXT:    xsmuldp 1, 5, 6
 ; CHECK-NEXT:    ld 3, 96(1)
 ; CHECK-NEXT:    stfd 0, 0(9)
 ; CHECK-NEXT:    stfd 0, 0(10)
-; CHECK-NEXT:    fmr 0, 3
-; CHECK-NEXT:    xsmsubadp 3, 1, 2
-; CHECK-NEXT:    xsmsubadp 0, 5, 6
-; CHECK-NEXT:    xsmuldp 4, 5, 6
-; CHECK-NEXT:    stfd 4, 0(3)
-; CHECK-NEXT:    xsadddp 1, 3, 0
+; CHECK-NEXT:    stfd 1, 0(3)
+; CHECK-NEXT:    xsnmsubadp 1, 3, 4
+; CHECK-NEXT:    xsnmsubadp 0, 3, 4
+; CHECK-NEXT:    xsadddp 1, 0, 1
 ; CHECK-NEXT:    blr
   %ab = fmul fast double %a, %b
   %cd = fmul fast double %c, %d

llvm/test/CodeGen/PowerPC/recipest.ll

@@ -177,12 +177,11 @@ define float @rsqrt_fmul_fmf(float %a, float %b, float %c) {
 ; CHECK-NEXT:    fmuls 1, 1, 0
 ; CHECK-NEXT:    fmadds 1, 1, 0, 4
 ; CHECK-NEXT:    fmuls 0, 0, 5
-; CHECK-NEXT:    fres 5, 2
+; CHECK-NEXT:    fmuls 0, 0, 1
+; CHECK-NEXT:    fres 1, 2
 ; CHECK-NEXT:    fmuls 4, 0, 1
-; CHECK-NEXT:    fmuls 4, 4, 5
-; CHECK-NEXT:    fmuls 2, 2, 4
-; CHECK-NEXT:    fmsubs 0, 0, 1, 2
-; CHECK-NEXT:    fmadds 0, 5, 0, 4
+; CHECK-NEXT:    fnmsubs 0, 2, 4, 0
+; CHECK-NEXT:    fmadds 0, 1, 0, 4
 ; CHECK-NEXT:    fmuls 1, 3, 0
 ; CHECK-NEXT:    blr
   %x = call fast float @llvm.sqrt.f32(float %a)