[CodeGen] Fix ShrinkWrap crash when FindIDom receives empty predecessor/successor list (#198995)

Michael-Chen-NJU · web-flow · commit fb6153a65039 · 2026-06-03T18:08:09.000+08:00
When using `-msave-restore` with `-ffixed-x5` on RISC-V, `canUseAsPrologue` returns false for all blocks because the save-restore prologue requires t0 (x5) which is reserved. This causes the shrink-wrap loop to keep searching for a valid save point, eventually reaching the entry block. On the next iteration, it calls FindIDom with the entry block's empty predecessor list, triggering an assertion in findNearestCommonDominator. Fix by returning nullptr from FindIDom when the block list is empty, which signals the caller to stop searching and give up the optimization. Fixes #166759
diff --git a/llvm/lib/CodeGen/ShrinkWrap.cpp b/llvm/lib/CodeGen/ShrinkWrap.cpp
@@ -384,6 +384,8 @@ bool ShrinkWrapImpl::useOrDefCSROrFI(const MachineInstr &MI, RegScavenger *RS,
 template <typename ListOfBBs, typename DominanceAnalysis>
 static MachineBasicBlock *FindIDom(MachineBasicBlock &Block, ListOfBBs BBs,
                                    DominanceAnalysis &Dom, bool Strict = true) {
+  if (BBs.begin() == BBs.end())
+    return Strict ? nullptr : &Block;
   MachineBasicBlock *IDom = Dom.findNearestCommonDominator(iterator_range(BBs));
   if (Strict && IDom == &Block)
     return nullptr;
diff --git a/llvm/test/CodeGen/RISCV/shrinkwrap-save-restore-fixed-reg.ll b/llvm/test/CodeGen/RISCV/shrinkwrap-save-restore-fixed-reg.ll
@@ -0,0 +1,38 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc -mtriple=riscv64 -mattr=+save-restore,+reserve-x5 -verify-machineinstrs < %s \
+; RUN:   | FileCheck %s
+
+; This test verifies that the shrink-wrap pass does not crash when the save
+; point reaches the entry block (which has no predecessors) while trying to find
+; a cheaper save point via FindIDom.
+
+@a = global i32 0, align 4
+
+define signext i32 @shrinkwrap_entry_no_preds() {
+; CHECK-LABEL: shrinkwrap_entry_no_preds:
+; CHECK:       # %bb.0: # %entry
+; CHECK-NEXT:    call t0, __riscv_save_0
+; CHECK-NEXT:    .cfi_def_cfa_offset 16
+; CHECK-NEXT:    .cfi_offset ra, -8
+; CHECK-NEXT:    lui a0, %hi(a)
+; CHECK-NEXT:    lw a0, %lo(a)(a0)
+; CHECK-NEXT:    beqz a0, .LBB0_2
+; CHECK-NEXT:  # %bb.1: # %if.then
+; CHECK-NEXT:    call b
+; CHECK-NEXT:  .LBB0_2: # %if.end
+; CHECK-NEXT:    li a0, 0
+; CHECK-NEXT:    tail __riscv_restore_0
+entry:
+  %0 = load i32, ptr @a, align 4
+  %tobool.not = icmp eq i32 %0, 0
+  br i1 %tobool.not, label %if.end, label %if.then
+
+if.then:
+  tail call void @b()
+  br label %if.end
+
+if.end:
+  ret i32 0
+}
+
+declare void @b()
