[SDAG] fold x * 0.0 at node creation time

In the motivating case from https://llvm.org/PR47517
we create a node that does not get constant folded
before getNegatedExpression is attempted from some
other node, and we crash.

By moving the fold into SelectionDAG::simplifyFPBinop(),
we get the constant fold sooner and avoid the problem.

Author: rotateright

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -13040,13 +13040,6 @@ SDValue DAGCombiner::visitFMUL(SDNode *N) {
   if (SDValue NewSel = foldBinOpIntoSelect(N))
     return NewSel;
 
-  if ((Options.NoNaNsFPMath && Options.NoSignedZerosFPMath) ||
-      (Flags.hasNoNaNs() && Flags.hasNoSignedZeros())) {
-    // fold (fmul A, 0) -> 0
-    if (N1CFP && N1CFP->isZero())
-      return N1;
-  }
-
   if (Options.UnsafeFPMath || Flags.hasAllowReassociation()) {
     // fmul (fmul X, C1), C2 -> fmul X, C1 * C2
     if (isConstantFPBuildVectorOrConstantFP(N1) &&

llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp

@@ -7475,6 +7475,11 @@ SDValue SelectionDAG::simplifyFPBinop(unsigned Opcode, SDValue X, SDValue Y,
     if (YC->getValueAPF().isExactlyValue(1.0))
       return X;
 
+  // X * 0.0 --> 0.0
+  if (Opcode == ISD::FMUL && Flags.hasNoNaNs() && Flags.hasNoSignedZeros())
+    if (YC->getValueAPF().isZero())
+      return getConstantFP(0.0, SDLoc(Y), Y.getValueType());
+
   return SDValue();
 }
 

llvm/test/CodeGen/ARM/softfp-constant-comparison.ll

@@ -9,24 +9,13 @@ target triple = "thumbv7em-arm-none-eabi"
 define hidden void @fn1() nounwind #0 {
 ; CHECK-LABEL: fn1:
 ; CHECK:       @ %bb.0: @ %entry
-; CHECK-NEXT:    .save {r7, lr}
-; CHECK-NEXT:    push {r7, lr}
-; CHECK-NEXT:    vldr d0, .LCPI0_0
-; CHECK-NEXT:    vmov r0, r1, d0
-; CHECK-NEXT:    mov r2, r0
-; CHECK-NEXT:    mov r3, r1
-; CHECK-NEXT:    bl __aeabi_dcmpeq
+; CHECK-NEXT:    movs r0, #1
 ; CHECK-NEXT:    cbnz r0, .LBB0_2
 ; CHECK-NEXT:    b .LBB0_1
 ; CHECK-NEXT:  .LBB0_1: @ %land.rhs
 ; CHECK-NEXT:    b .LBB0_2
 ; CHECK-NEXT:  .LBB0_2: @ %land.end
-; CHECK-NEXT:    pop {r7, pc}
-; CHECK-NEXT:    .p2align 3
-; CHECK-NEXT:  @ %bb.3:
-; CHECK-NEXT:  .LCPI0_0:
-; CHECK-NEXT:    .long 0 @ double 0
-; CHECK-NEXT:    .long 0
+; CHECK-NEXT:    bx lr
 entry:
   %0 = load i32, i32* @a, align 4
   %conv = sitofp i32 %0 to double

llvm/test/CodeGen/X86/fmul-combines.ll

@@ -261,3 +261,22 @@ define <4 x float> @fmul_fneg_fneg_v4f32(<4 x float> %x, <4 x float> %y) {
   %mul = fmul <4 x float> %x.neg, %y.neg
   ret <4 x float> %mul
 }
+
+; PR47517 - this could crash if we create 'fmul x, 0.0' nodes
+; that do not constant fold in a particular order.
+
+define float @getNegatedExpression_crash(float* %p) {
+; CHECK-LABEL: getNegatedExpression_crash:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    movl $0, (%rdi)
+; CHECK-NEXT:    xorps %xmm0, %xmm0
+; CHECK-NEXT:    retq
+  store float 0.0, float* %p, align 1
+  %real = load float, float* %p, align 1
+  %r2 = fmul fast float %real, %real
+  %t1 = fmul fast float %real, 42.0
+  %t2 = fmul fast float %real, %t1
+  %mul_ac56 = fmul fast float %t2, %t1
+  %mul_ac72 = fmul fast float %r2, %mul_ac56
+  ret float %mul_ac72
+}