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[MIRPrinter] Print empty successor lists when they cannot be guessed 

Unreachable blocks in the machine instr representation are these
weird empty blocks with no successors.
The MIR printer used to not print empty lists of successors. However,
the MIR parser now treats non-printed list of successors as "please
guess it for me". As a result, the parser tries to guess the list of
successors and given the block is empty, just assumes it falls through
the next block (if any).

For instance, the following test case used to fail the verifier.
The MIR printer would print
          entry
         /      \
    true (def)   false (no list of successors)
        |
  split.true (use)

The MIR parser would understand this:
          entry
         /      \
    true (def)   false
        |        /  <-- invalid edge
  split.true (use)

Because of the invalid edge, we get the "def does not
dominate all uses" error.

The fix consists in printing empty successor lists, so that the parser
knows what to do for unreachable blocks.

rdar://problem/34022159

llvm-svn: 313685
This commit is contained in:
Quentin Colombet 2017-09-19 21:55:51 +00:00
parent 21a2aa7203
commit 7fdaa5e641
2 changed files with 49 additions and 2 deletions
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3
llvm/lib/CodeGen/MIRPrinter.cpp
View File

@ -598,8 +598,7 @@ void MIPrinter::print(const MachineBasicBlock &MBB) {
bool HasLineAttributes = false;
// Print the successors
bool canPredictProbs = canPredictBranchProbabilities(MBB);
if (!MBB.succ_empty() && (!SimplifyMIR || !canPredictProbs ||
!canPredictSuccessors(MBB))) {
if (!SimplifyMIR || !canPredictProbs || !canPredictSuccessors(MBB)) {
OS.indent(2) << "successors: ";
for (auto I = MBB.succ_begin(), E = MBB.succ_end(); I != E; ++I) {
if (I != MBB.succ_begin())

48
llvm/test/CodeGen/MIR/X86/unreachable_block.ll Normal file
View File

@ -0,0 +1,48 @@
; RUN: llc -mtriple x86_64-- -stop-before peephole-opt -o %t.mir %s
; RUN: llc -mtriple x86_64-- -run-pass none %t.mir -verify-machineinstrs -o - | FileCheck %s
; Unreachable blocks in the machine instr representation are these
; weird empty blocks with no successors.
; The MIR printer used to not print empty lists of successors. However,
; the MIR parser now treats non-printed list of successors as "please
; guess it for me". As a result, the parser tries to guess the list of
; successors and given the block is empty, just assumes it falls through
; the next block.
;
; The following test case used to fail the verifier because the false
; path ended up falling through split.true and now, the definition of
; %v does not dominate all its uses.
; Indeed, we go from the following CFG:
; entry
; / \
; true (def) false
; |
; split.true (use)
;
; To this one:
; entry
; / \
; true (def) false
; | / <-- invalid edge
; split.true (use)
;
; Because of the invalid edge, we get the "def does not
; dominate all uses" error.
;
; CHECK-LABEL: name: foo
; CHECK-LABEL: bb.{{[0-9]+}}.false:
; CHECK-NEXT: successors:
; CHECK-NOT: %bb.{{[0-9]+}}.split.true
; CHECK-LABEL: bb.{{[0-9]+}}.split.true:
define void @foo(i32* %bar) {
br i1 undef, label %true, label %false
true:
%v = load i32, i32* %bar
br label %split.true
false:
unreachable
split.true:
%vInc = add i32 %v, 1
store i32 %vInc, i32* %bar
ret void
}