[llvm] 3562ad3 - BPF: implement isLegalAddressingMode() properly

[Yonghong Song via llvm-commits]

Author: Yonghong Song
Date: 2021-09-30T16:41:15-07:00
New Revision: 3562ad3ebeb29d0cf320bfcf51f64372e9b37c95

URL: https://github.com/llvm/llvm-project/commit/3562ad3ebeb29d0cf320bfcf51f64372e9b37c95
DIFF: https://github.com/llvm/llvm-project/commit/3562ad3ebeb29d0cf320bfcf51f64372e9b37c95.diff

LOG: BPF: implement isLegalAddressingMode() properly

Latest upstream llvm caused the kernel bpf selftest emitting the
following warnings:

  In file included from progs/profiler3.c:6:
  progs/profiler.inc.h:489:2: warning: loop not unrolled:
    the optimizer was unable to perform the requested transformation;
    the transformation might be disabled or specified as part of an unsupported
    transformation ordering [-Wpass-failed=transform-warning]
          for (int i = 0; i < MAX_PATH_DEPTH; i++) {
          ^

Further bisecting shows this SimplifyCFG patch ([1]) changed
the condition on how to fold branch to common dest. This caused
some unroll pragma is not honored in selftests/bpf.

The patch [1] test getUserCost() as the condition to
perform the certain basic block folding transformation.
For the above example, before the loop unroll pass, the control flow
looks like:
    cond_block:
       branch target: body_block, cleanup_block
    body_block:
       branch target: cleanup_block, end_block
    end_block:
       branch target: cleanup_block, end10_block
    end10_block:
       %add.ptr = getelementptr i8, i8* %payload.addr.0, i64 %call2
       %inc = add nuw nsw i32 %i.0, 1
       branch target: cond_block

In the above, %call2 is an unknown scalar.

Before patch [1], end10_block will be folded into end_block, forming
the code like
    cond_block:
       branch target: body_block, cleanup_block
    body_block:
       branch target: cleanup_block, end_block
    end_block:
       branch target: cleanup_block, cond_block
and the compiler is happy to perform unrolling.

With patch [1], getUserCost(), which calls getGEPCost(), which calls
isLegalAddressingMode() in TargetLoweringBase.cpp, considers IR
  %add.ptr = getelementptr i8, i8* %payload.addr.0, i64 %call2
is free, so the above basic block folding transformation is not performed
and unrolling does not happen.

For BPF target, the IR
  %add.ptr = getelementptr i8, i8* %payload.addr.0, i64 %call2
is not free and we don't have ld/st instruction address with 'r+r' mode.

This patch implemented a BPF hook for isLegalAddressingMode(), which is
identical to Mips isLegalAddressingMode() implementation where
the address pattern like 'r+r', 'r+r+i' or '2*r' are not allowed.
With testing kernel bpf selftests, all loop not unrolled warnings
are gone and all selftests run successfully.

  [1] https://reviews.llvm.org/D108837

Differential Revision: https://reviews.llvm.org/D110789

Added: 
    

Modified: 
    llvm/lib/Target/BPF/BPFISelLowering.cpp
    llvm/lib/Target/BPF/BPFISelLowering.h

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Target/BPF/BPFISelLowering.cpp b/llvm/lib/Target/BPF/BPFISelLowering.cpp
index c543dfcfca953..85c74b9a2425a 100644
--- a/llvm/lib/Target/BPF/BPFISelLowering.cpp
+++ b/llvm/lib/Target/BPF/BPFISelLowering.cpp
@@ -859,3 +859,25 @@ MVT BPFTargetLowering::getScalarShiftAmountTy(const DataLayout &DL,
                                               EVT VT) const {
   return (getHasAlu32() && VT == MVT::i32) ? MVT::i32 : MVT::i64;
 }
+
+bool BPFTargetLowering::isLegalAddressingMode(const DataLayout &DL,
+                                              const AddrMode &AM, Type *Ty,
+                                              unsigned AS,
+                                              Instruction *I) const {
+  // No global is ever allowed as a base.
+  if (AM.BaseGV)
+    return false;
+
+  switch (AM.Scale) {
+  case 0: // "r+i" or just "i", depending on HasBaseReg.
+    break;
+  case 1:
+    if (!AM.HasBaseReg) // allow "r+i".
+      break;
+    return false; // disallow "r+r" or "r+r+i".
+  default:
+    return false;
+  }
+
+  return true;
+}

diff  --git a/llvm/lib/Target/BPF/BPFISelLowering.h b/llvm/lib/Target/BPF/BPFISelLowering.h
index d5007425a7f8d..dcc53019db752 100644
--- a/llvm/lib/Target/BPF/BPFISelLowering.h
+++ b/llvm/lib/Target/BPF/BPFISelLowering.h
@@ -130,6 +130,10 @@ class BPFTargetLowering : public TargetLowering {
     return false;
   }
 
+  bool isLegalAddressingMode(const DataLayout &DL, const AddrMode &AM,
+                             Type *Ty, unsigned AS,
+                             Instruction *I = nullptr) const override;
+
   // isTruncateFree - Return true if it's free to truncate a value of
   // type Ty1 to type Ty2. e.g. On BPF at alu32 mode, it's free to truncate
   // a i64 value in register R1 to i32 by referencing its sub-register W1.