[DAGCombiner] Remove overzealous assertion when folding assert+trunc+assert (PR55846)

[nikic]

Summary

These assert that there are no "useless" assertzext/assertsext nodes (that assert a wider width than a following trunc), but I don't think there is anything preventing such nodes from reaching this code. I don't think the assertion is relevant for correctness of this transform either -- if such an assert is present, then the other one will always be to a smaller width, and we'll pick that one. The assertion dates back to D37017.

Fixes https://github.com/llvm/llvm-project/issues/55846.

[uabelho]

Fixes https://github.com/llvm/llvm-project/issues/55848 as well.

Thanks!

[spatel]

LGTM

llvm/test/CodeGen/X86/pr55846.ll

5

Put a comment on this test, so we don't have to dig for its purpose?

; After legalization, this could be: "i8 truncate (i64 AssertZext X, Type: i9)"
; The AssertZext does not add information, so it should be eliminated,
; but that must not trigger a compile-time assert.

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

SDValue DAGCombiner::visitAssertExt(SDNode *N) {
 
   if (N0.getOpcode() == ISD::TRUNCATE && N0.hasOneUse() &&
       N0.getOperand(0).getOpcode() == Opcode) {
     // We have an assert, truncate, assert sandwich. Make one stronger assert
     // by asserting on the smallest asserted type to the larger source type.
     // This eliminates the later assert:
     // assert (trunc (assert X, i8) to iN), i1 --> trunc (assert X, i1) to iN
     // assert (trunc (assert X, i1) to iN), i8 --> trunc (assert X, i1) to iN
+    SDLoc DL(N);
     SDValue BigA = N0.getOperand(0);
     EVT BigA_AssertVT = cast<VTSDNode>(BigA.getOperand(1))->getVT();
-    assert(BigA_AssertVT.bitsLE(N0.getValueType()) &&
-           "Asserting zero/sign-extended bits to a type larger than the "
-           "truncated destination does not provide information");
-
-    SDLoc DL(N);
     EVT MinAssertVT = AssertVT.bitsLT(BigA_AssertVT) ? AssertVT : BigA_AssertVT;
     SDValue MinAssertVTVal = DAG.getValueType(MinAssertVT);
     SDValue NewAssert = DAG.getNode(Opcode, DL, BigA.getValueType(),
                                     BigA.getOperand(0), MinAssertVTVal);
     return DAG.getNode(ISD::TRUNCATE, DL, N->getValueType(0), NewAssert);
   }
 
   // If we have (AssertZext (truncate (AssertSext X, iX)), iY) and Y is smaller
   // than X. Just move the AssertZext in front of the truncate and drop the
   // AssertSExt.
   if (N0.getOpcode() == ISD::TRUNCATE && N0.hasOneUse() &&
       N0.getOperand(0).getOpcode() == ISD::AssertSext &&
       Opcode == ISD::AssertZext) {
     SDValue BigA = N0.getOperand(0);
     EVT BigA_AssertVT = cast<VTSDNode>(BigA.getOperand(1))->getVT();
-    assert(BigA_AssertVT.bitsLE(N0.getValueType()) &&
-           "Asserting zero/sign-extended bits to a type larger than the "
-           "truncated destination does not provide information");
-
     if (AssertVT.bitsLT(BigA_AssertVT)) {
       SDLoc DL(N);
       SDValue NewAssert = DAG.getNode(Opcode, DL, BigA.getValueType(),
                                       BigA.getOperand(0), N1);
       return DAG.getNode(ISD::TRUNCATE, DL, N->getValueType(0), NewAssert);
     }
   }
 

llvm/test/CodeGen/X86/pr55846.ll

+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc -mtriple=x86_64-unknown-linux-gnu < %s | FileCheck %s
+
+; After legalization, this could be: "i8 truncate (i64 AssertZext X, Type: i9)"
+; The AssertZext does not add information, so it should be eliminated,
+; but that must not trigger a compile-time assert.
+
+define void @test(i64* %p) {
+; CHECK-LABEL: test:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    movl $256, %eax # imm = 0x100
+; CHECK-NEXT:    movq %rax, (%rdi)
+; CHECK-NEXT:    retq
+  %sel = select i1 true, i64 256, i64 0
+  br label %bb2
+
+bb2:
+  store i64 %sel, i64* %p, align 4
+  %p.bc = bitcast i64* %p to <2 x i1>*
+  %load = load <2 x i1>, <2 x i1>* %p.bc, align 1
+  br label %bb3
+
+bb3:
+  %use = add <2 x i1> %load, zeroinitializer
+  ret void
+}