[MachineSink] Bail out if BreakPHIEdge is not consistent across iterations

[lkail]

Summary

Suppose we have CFG

                +-----------------+
                |%bb.0:           |
                |%r0, %r1 = OP ...|
         +------+-------------+---+
         |                    |
         |                    |
         |                    |
         |                    |
+--------v-------+            |
|%bb.1:          |            |
|%r2 = OP ...    |            |
+----------------+---+        |
                     |        |
                     |        |
                     |        |
                     |        |
                +----v--------v----+
                |%bb.2:            |
                |%r3 = PHI %r0, %r2|
                |USE %r1           |
                +------------------+

When testing if %bb.2 is a proper sink point for %r0, BreakPHIEdge is set true, a new BB is supposed to be added between bb.0 and bb.2. Then %r1 is also tested against %bb.2, however BreakPHIEdge is set false. This leads to sinking %r0, %r1 = OP ... after %r3 = PHI %r0, %r2 in %bb.2, which is not right. This patch checks consistency of BreakPHIEdge between adjacent iteration and bail out if it's inconsistent.

Fixes https://bugs.llvm.org/show_bug.cgi?id=45557.

[lkail]

Already fixed by https://reviews.llvm.org/D78087, but not landed now.

llvm/lib/CodeGen/MachineSink.cpp

if (Register::isPhysicalRegister(Reg)) {
       if (!TII->isSafeToMoveRegClassDefs(MRI->getRegClass(Reg)))
         return nullptr;
 
       // Virtual register defs can only be sunk if all their uses are in blocks
       // dominated by one of the successors.
       if (SuccToSinkTo) {
         // If a previous operand picked a block to sink to, then this operand
         // must be sinkable to the same block.
-        bool LocalUse = false;
-        if (!AllUsesDominatedByBlock(Reg, SuccToSinkTo, MBB,
-                                     BreakPHIEdge, LocalUse))
+        bool LocalUse = false, PrevBreakPHIEdge = BreakPHIEdge;
+        if (!AllUsesDominatedByBlock(Reg, SuccToSinkTo, MBB, BreakPHIEdge,
+                                     LocalUse) ||
+            BreakPHIEdge != PrevBreakPHIEdge)
           return nullptr;
 
         continue;
       }
 
       // Otherwise, we should look at all the successors and decide which one
       // we should sink to. If we have reliable block frequency information
       // (frequency != 0) available, give successors with smaller frequencies

llvm/test/CodeGen/PowerPC/pr45557.mir

+# RUN: llc -mtriple=powerpc -verify-machineinstrs -simplify-mir \
+# RUN:   --run-pass=machine-sink %s -o - | FileCheck %s
+
+--- |
+  @__const = external dso_local unnamed_addr constant [2 x i32], align 8
+  define void @c() {
+  entry:
+    unreachable
+  }
+...
+---
+name:            c
+alignment:       4
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    %3:gprc_and_gprc_nor0 = LIS target-flags(ppc-ha) @__const
+    %0:gprc, %4:gprc_and_gprc_nor0 = LWZU target-flags(ppc-lo) @__const, %3 :: (load 4 from `i64* bitcast ([2 x i32]* @__const to i64*)`, align 8)
+    %5:crbitrc = IMPLICIT_DEF
+    BC killed %5, %bb.2
+    B %bb.1
+
+  bb.1:
+    %6:gprc = LI 0
+
+  bb.2:
+    %2:gprc = PHI %0, %bb.0, %6, %bb.1
+    %1:gprc = LWZ 4, killed %4 :: (dereferenceable load 4 from `i64* bitcast ([2 x i32]* @__const to i64*)` + 4, align 8)
+    BLR implicit $lr, implicit $rm
+...
+# CHECK-LABEL: bb.2:
+# CHECK-NOT:     LWZU