[X86][FastISel] Fix with.overflow eflags clobber (PR49587)

If the successor block has a phi node, then additional moves may
be inserted into predecessors, which may clobber eflags. Don't try
to fold the with.overflow result into the branch in that case.

This is done by explicitly checking for any phis in successor
blocks, not sure if there's some more principled way to address
this. Other fused compare and branch patterns avoid the issue by
emitting the comparison when handling the branch, so that no
instructions may be inserted in between. In this case, the
with.overflow call is emitted separately (and I don't think this
is avoidable, as it will generally have at least two users).

Fixes https://bugs.llvm.org/show_bug.cgi?id=49587.

Differential Revision: https://reviews.llvm.org/D98600

(cherry picked from commit 7669455)

Author: nikic

llvm/lib/Target/X86/X86FastISel.cpp

@@ -284,6 +284,14 @@ bool X86FastISel::foldX86XALUIntrinsic(X86::CondCode &CC, const Instruction *I,
       return false;
   }
 
+  // Make sure no potentially eflags clobbering phi moves can be inserted in
+  // between.
+  auto HasPhis = [](const BasicBlock *Succ) {
+    return !llvm::empty(Succ->phis());
+  };
+  if (I->isTerminator() && llvm::any_of(successors(I), HasPhis))
+    return false;
+
   CC = TmpCC;
   return true;
 }

llvm/test/CodeGen/X86/pr49587.ll

@@ -5,10 +5,11 @@ define i32 @test(i64 %arg) nounwind {
 ; CHECK-LABEL: test:
 ; CHECK:       # %bb.0: # %entry
 ; CHECK-NEXT:    subq $1, %rdi
-; CHECK-NEXT:    setb %al
+; CHECK-NEXT:    setb %cl
 ; CHECK-NEXT:    xorl %eax, %eax
+; CHECK-NEXT:    testb $1, %cl
 ; CHECK-NEXT:    movl %eax, {{[-0-9]+}}(%r{{[sb]}}p) # 4-byte Spill
-; CHECK-NEXT:    jb .LBB0_2
+; CHECK-NEXT:    jne .LBB0_2
 ; CHECK-NEXT:  # %bb.1: # %no_overflow
 ; CHECK-NEXT:    movl $1, %eax
 ; CHECK-NEXT:    movl %eax, {{[-0-9]+}}(%r{{[sb]}}p) # 4-byte Spill