[InstCombine] Fix div handling

[skatkov]

Summary

When we optimize select basing on fact that div by 0 is undef
we should not traverse the instruction which are not guaranteed to
transfer execution to next instruction. Guard intrinsic is an example.

[spatel]

test/Transforms/InstCombine/sdiv-guard.ll

1

This test already passes without the patch. Please create a test that shows the diff and use utils/update_test_checks.py to generate checks for it.

Also, we shouldn't need -inline to show a failure in instcombine.

[skatkov]

Please take a look.

[skatkov]

test/Transforms/InstCombine/sdiv-guard.ll

1

Test actually shows the diff. Without this patch it will transform guard condition to %X != 0 which is incorrect because if X is not 0 and flag is false gaurd should be triggered.

[spatel]

LGTM - see inline comments.

lib/Transforms/InstCombine/InstCombineMulDivRem.cpp

586

Typo:
"If we found an instruction that we can't assume will return..."

test/Transforms/InstCombine/sdiv-guard.ll

1

Ah, the assertions were just not strong enough then. I committed the baseline test here:
rL333756

Please remove the 'FIXME' and update/rebase the patch.

lib/Transforms/InstCombine/InstCombineMulDivRem.cpp

bool InstCombiner::simplifyDivRemOfSelectWithZeroOp(BinaryOperator &I) {
   if (SI->use_empty() && SelectCond->hasOneUse())
     return true;
 
   // Scan the current block backward, looking for other uses of SI.
   BasicBlock::iterator BBI = I.getIterator(), BBFront = I.getParent()->begin();
   Type *CondTy = SelectCond->getType();
   while (BBI != BBFront) {
     --BBI;
-    // If we found a call to a function, we can't assume it will return, so
+    // If we found an instruction that we can't assume it will return, so
     // information from below it cannot be propagated above it.
-    if (isa<CallInst>(BBI) && !isa<IntrinsicInst>(BBI))
+    if (!isGuaranteedToTransferExecutionToSuccessor(&*BBI))
       break;
 
     // Replace uses of the select or its condition with the known values.
     for (Instruction::op_iterator I = BBI->op_begin(), E = BBI->op_end();
          I != E; ++I) {
       if (*I == SI) {
         *I = SI->getOperand(NonNullOperand);
         Worklist.Add(&*BBI);

test/Transforms/InstCombine/sdiv-guard.ll

+; RUN: opt < %s -instcombine -inline -S | FileCheck %s
+
+declare void @llvm.experimental.guard(i1, ...)
+
+; Regression test. If %flag is false then %s == 0 and guard should be triggered.
+define i32 @a(i1 %flag, i32 %X) nounwind readnone {
+entry:
+;CHECK: call
+;CHECK: sdiv i32 100, %X
+  %s = select i1 %flag, i32 %X, i32 0
+  %cmp = icmp ne i32 %s, 0
+  call void(i1, ...) @llvm.experimental.guard( i1 %cmp )[ "deopt"() ]
+  %r = sdiv i32 100, %s
+  ret i32 %r
+}