[RISCV] Fix std::advance slowness

Summary:
It seems std::advance template is treating "-MFI.getCalleeSavedInfo().size()"
as a large unsigned value", causing slowness.

Thanks to Henrik Gustafsson for reporting the issue.

Reviewers: asb

Reviewed By: asb

Subscribers: llvm-commits, rbar, johnrusso, simoncook, sabuasal, niosHD, kito-cheng, shiva0217, zzheng, edward-jones, mgrang, rogfer01, MartinMosbeck, brucehoult, the_o, rkruppe, PkmX, jocewei, asb

Differential Revision: https://reviews.llvm.org/D51148

llvm-svn: 340669

1 files changed, 1 insertions, 2 deletions

diff --git a/llvm/lib/Target/RISCV/RISCVFrameLowering.cpp b/llvm/lib/Target/RISCV/RISCVFrameLowering.cpp
index a816028f9d8..74417899c8d 100644
--- a/llvm/lib/Target/RISCV/RISCVFrameLowering.cpp
+++ b/llvm/lib/Target/RISCV/RISCVFrameLowering.cpp
@@ -148,8 +148,7 @@ void RISCVFrameLowering::emitEpilogue(MachineFunction &MF,
   // Skip to before the restores of callee-saved registers
   // FIXME: assumes exactly one instruction is used to restore each
   // callee-saved register.
-  MachineBasicBlock::iterator LastFrameDestroy = MBBI;
-  std::advance(LastFrameDestroy, -MFI.getCalleeSavedInfo().size());
+  auto LastFrameDestroy = std::prev(MBBI, MFI.getCalleeSavedInfo().size());
 
   uint64_t StackSize = MFI.getStackSize();