IfConversion: Add assertions that both sides of a diamond don't pred-clobber.

[iteratee]

Summary

One side of a diamond may end with a predicate clobbering instruction.
That side of the diamond has to be if-converted second. Both sides can't
clobber the predicate or the ifconversion is invalid. This is checked
elsewhere, but add an assert as a safety check. NFC

[iteratee]

Assertion split into separate revision.

[davidxl]

lgtm

[Eugene.Zelenko]

Committed in rL280517. Please specify "Differential revision: <URL>" in commit message.

[iteratee]

In D24181#561469, @Eugene.Zelenko wrote:

Committed in rL280517. Please specify "Differential revision: <URL>" in commit message.

Actually this was committed in RL280518. Sorry about the URL missing.

[iteratee]

committed in rL280518

lib/CodeGen/IfConversion.cpp

if (!TailBB) {
     if (blockAlwaysFallThrough(TrueBBI))
       TailBB = FalseBBI.TrueBB;
     assert((TailBB || !TrueBBI.IsBrAnalyzable) && "Unexpected!");
   }
 
   if (!IfConvertDiamondCommon(
       BBI, TrueBBI, FalseBBI,
       NumDups1, NumDups2,
-      TrueBBI.ClobbersPred, FalseBBI.ClobbersPred,
+      TClobbersPred, FClobbersPred,
       /* RemoveBranch */ TrueBBI.IsBrAnalyzable,
       /* MergeAddEdges */ TailBB == nullptr))
     return false;
 
   // If the if-converted block falls through or unconditionally branches into
   // the tail block, and the tail block does not have other predecessors, then
   // fold the tail block in as well. Otherwise, unless it falls through to the
   // tail, add a unconditional branch to it.

test/CodeGen/Thumb2/ifcvt-rescan-diamonds.ll

+; RUN: llc -O2 -o - %s | FileCheck %s
+target datalayout = "e-m:e-p:32:32-i64:64-v128:64:128-a:0:32-n32-S64"
+target triple = "thumbv8-unknown-linux-gnueabihf"
+
+; This is a tricky test case.
+; The point of it is to create a diamond where both the true block and the
+; false block clobber the predicate when viewed as a whole, but only one of them
+; clobbers the predicate when considering the instructions they share.
+
+; Function Attrs: nounwind
+define void @BN_kronecker(i1 %a, i32 %b) #0 {
+entry:
+  br label %while.cond38
+
+while.cond38:                                     ; preds = %if.end111, %entry
+  %cmp79 = icmp eq i32 0, 0
+  br i1 %a, label %cond.true77, label %cond.false87
+
+; CHECK: %cond.true77
+; CHECK-NEXT: @ in Loop
+; CHECK-NEXT: cmp.w {{r[0-9]+}}, #0
+; CHECK-NEXT: it eq
+; CHECK-NEXT: ldreq
+; CHECK-NEXT: it ne
+; CHECK-NEXT: movsne
+; CHECK-NEXT: mvns
+; CHECK-NEXT: b
+cond.true77:                                      ; preds = %while.cond38
+  br i1 %cmp79, label %cond.end84, label %cond.false81
+
+cond.false81:                                     ; preds = %cond.true77
+  %0 = load i32, i32* null, align 4
+  br label %cond.end84
+
+cond.end84:                                       ; preds = %cond.false81, %cond.true77
+  %cond85 = phi i32 [ %0, %cond.false81 ], [ 0, %cond.true77 ]
+  %neg86 = xor i32 %cond85, -1
+  br label %cond.false101
+
+cond.false87:                                     ; preds = %while.cond38
+  br i1 %cmp79, label %cond.false101, label %cond.false91
+
+cond.false91:                                     ; preds = %cond.false87
+  br label %cond.false101
+
+cond.false101:                                    ; preds = %cond.false91, %cond.false87, %cond.end84
+  %cond97 = phi i32 [ %neg86, %cond.end84 ], [ %b, %cond.false91 ], [ 0, %cond.false87 ]
+  %1 = load i32, i32* null, align 4
+  %and106 = and i32 %cond97, %1
+  %and107 = and i32 %and106, 2
+  %tobool108 = icmp ne i32 %and107, 0
+  br i1 %tobool108, label %if.then109, label %if.end111
+
+if.then109:                                       ; preds = %cond.false101
+  store i32 0, i32* undef, align 4
+  br label %if.end111
+
+if.end111:                                        ; preds = %if.then109, %cond.false101
+  %tobool113 = icmp ne i32 0, 0
+  br i1 %tobool113, label %while.cond38, label %end
+
+end:                                              ; preds = %if.end111
+  ret void
+}
+
+attributes #0 = { nounwind }