[MachineTraceMetrics] Pick the trace successor for an entry block

[asi-sc]

Summary

We generate erroneous trace for a basic block if it does not have at least one
predecessor when MinInstr strategy is used. Currently only this strategy is
implemented, so we always have a wrong trace for any entry block. This results in
wrong instructions heights calculation and also leads to wrong critical path.

The described behavior is demonstrated on a simple test. It shows that early
if-conv pass makes wrong decisions due to incorrectly calculated critical path
lenght.

[jroelofs]

LGTM

llvm/lib/CodeGen/MachineTraceMetrics.cpp

for (const MachineBasicBlock *Pred : MBB->predecessors()) {
     }
   }
   return Best;
 }
 
 // Select the preferred successor for MBB.
 const MachineBasicBlock*
 MinInstrCountEnsemble::pickTraceSucc(const MachineBasicBlock *MBB) {
-  if (MBB->pred_empty())
+  if (MBB->succ_empty())
     return nullptr;
   const MachineLoop *CurLoop = getLoopFor(MBB);
   const MachineBasicBlock *Best = nullptr;
   unsigned BestHeight = 0;
   for (const MachineBasicBlock *Succ : MBB->successors()) {
     // Don't consider back-edges.
     if (CurLoop && Succ == CurLoop->getHeader())
       continue;

llvm/test/CodeGen/X86/machine-trace-metrics-entryBB-critpath.ll

 ; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
 ; RUN: llc -mtriple=x86_64-unknown-unknown -x86-early-ifcvt -debug-only=early-ifcvt < %s 2>&1 | FileCheck %s
 
 ; This test case ensures that machine trace metrics properly calculates
 ; instructions heights and the trace's critical path for the entry block when
 ; MinInstr strategy is used. The behavior is demonstrated on early if conversion
 ; pass.
 
-; CHECK: TBB: MinInstr trace %bb.0 --> %bb.0 --> %bb.0: 4 instrs. 1 cycles.
+; CHECK: TBB: MinInstr trace %bb.0 --> %bb.0 --> %bb.2: 8 instrs. 30 cycles.
 ; CHECK: %bb.0
+; CHECK:     -> %bb.2
 
 ; CHECK: FBB: MinInstr trace %bb.0 --> %bb.1 --> %bb.2: 10 instrs. 32 cycles.
 ; CHECK: %bb.1 <- %bb.0
 ; CHECK:     -> %bb.2
 
-; CHECK: Resource length 10, minimal critical path 1
-; CHECK: Not enough available ILP.
+; CHECK: Resource length 10, minimal critical path 30
+; CHECK: If-converting
 
 define i32 @_Z3fooiidd(i32 %a, i32 %b, double %d, double %e) #0 {
 ; CHECK-LABEL: _Z3fooiidd:
 ; CHECK:       # %bb.0: # %entry
-; CHECK-NEXT:    movl %esi, %eax
-; CHECK-NEXT:    addl %edi, %eax
-; CHECK-NEXT:    cmpl $3, %edi
-; CHECK-NEXT:    jl .LBB0_2
-; CHECK-NEXT:  # %bb.1: # %if.then
-; CHECK-NEXT:    cvttsd2si %xmm0, %ecx
+; CHECK-NEXT:    # kill: def $esi killed $esi def $rsi
+; CHECK-NEXT:    # kill: def $edi killed $edi def $rdi
+; CHECK-NEXT:    leal (%rsi,%rdi), %ecx
+; CHECK-NEXT:    cvttsd2si %xmm0, %eax
 ; CHECK-NEXT:    addl %ecx, %eax
-; CHECK-NEXT:  .LBB0_2: # %if.end
+; CHECK-NEXT:    cmpl $3, %edi
+; CHECK-NEXT:    cmovll %ecx, %eax
 ; CHECK-NEXT:    cvttsd2si %xmm1, %ecx
 ; CHECK-NEXT:    cltd
 ; CHECK-NEXT:    idivl %ecx
 ; CHECK-NEXT:    retq
 entry:
   %add = add nsw i32 %b, %a
   %cmp = icmp sgt i32 %a, 2
   br i1 %cmp, label %if.then, label %if.end