[X86] Do not combine LRINT and TRUNC (llvm#125848) (swiftlang#10385)

Per to discussions in llvm#125324, most participants are opposed to this
optimization. So remove the combination to address the concerns.

Fixes llvm#125324

(cherry picked from commit 8c222c1)

Co-authored-by: Phoebe Wang <[email protected]>

Author: juliannagele

llvm/lib/Target/X86/X86ISelLowering.cpp

@@ -52654,11 +52654,6 @@ static SDValue combineTruncate(SDNode *N, SelectionDAG &DAG,
       return DAG.getNode(X86ISD::MMX_MOVD2W, DL, MVT::i32, BCSrc);
   }
 
-  // Try to combine (trunc (vNi64 (lrint x))) to (vNi32 (lrint x)).
-  if (Src.getOpcode() == ISD::LRINT && VT.getScalarType() == MVT::i32 &&
-      Src.hasOneUse())
-    return DAG.getNode(ISD::LRINT, DL, VT, Src.getOperand(0));
-
   return SDValue();
 }
 

llvm/test/CodeGen/X86/lrint-conv-i64.ll

@@ -45,6 +45,24 @@ entry:
   ret i64 %0
 }
 
+define i32 @PR125324(float %x) {
+; SSE-LABEL: PR125324:
+; SSE:       # %bb.0: # %entry
+; SSE-NEXT:    cvtss2si %xmm0, %rax
+; SSE-NEXT:    # kill: def $eax killed $eax killed $rax
+; SSE-NEXT:    retq
+;
+; AVX-LABEL: PR125324:
+; AVX:       # %bb.0: # %entry
+; AVX-NEXT:    vcvtss2si %xmm0, %rax
+; AVX-NEXT:    # kill: def $eax killed $eax killed $rax
+; AVX-NEXT:    retq
+entry:
+  %0 = tail call i64 @llvm.lrint.i64.f32(float %x)
+  %1 = trunc i64 %0 to i32
+  ret i32 %1
+}
+
 declare i64 @llvm.lrint.i64.f32(float) nounwind readnone
 declare i64 @llvm.lrint.i64.f64(double) nounwind readnone
 declare i64 @llvm.lrint.i64.f80(x86_fp80) nounwind readnone