StackLifetime: Remove asserts for multiple lifetime intrinsics.

[pcc]

Summary

According to the langref, it is valid to have multiple consecutive
lifetime start or end intrinsics on the same object.

For llvm.lifetime.start:
"If ptr [...] is a stack object that is already alive, it simply
fills all bytes of the object with poison."

For llvm.lifetime.end:
"Calling llvm.lifetime.end on an already dead alloca is no-op."

However, we currently fail an assertion in such cases. I've observed
the assertion failure when the loop vectorization pass duplicates
the intrinsic.

We can conservatively handle these intrinsics by ignoring all but
the first one, which can be implemented by removing the assertions.

[eugenis]

Please update test/Analysis/StackSafetyAnalysis/lifetime.ll instead, and it would be good to extend it with testcases for liveness between two start and two end calls.

[pcc]

Test this in test/Analysis/StackSafetyAnalysis/lifetime.ll instead

[eugenis]

LGTM

llvm/lib/Analysis/StackLifetime.cpp

for (auto IT : BlockLiveness) {
     }
 
     for (auto &It : BBMarkers[BB]) {
       unsigned InstNo = It.first;
       bool IsStart = It.second.IsStart;
       unsigned AllocaNo = It.second.AllocaNo;
 
       if (IsStart) {
-        assert(!Started.test(AllocaNo) || Start[AllocaNo] == BBStart);
         if (!Started.test(AllocaNo)) {
           Started.set(AllocaNo);
           Ended.reset(AllocaNo);
           Start[AllocaNo] = InstNo;
         }
       } else {
-        assert(!Ended.test(AllocaNo));
         if (Started.test(AllocaNo)) {
           LiveRanges[AllocaNo].addRange(Start[AllocaNo], InstNo);
           Started.reset(AllocaNo);
         }
         Ended.set(AllocaNo);
       }
     }
 

llvm/test/Analysis/StackSafetyAnalysis/lifetime.ll

; MUST-NEXT: Alive: <>
   call void @llvm.lifetime.end.p0i32(i64 15, i32* %x2)
 ; CHECK: call void @llvm.lifetime.end.p0i32(i64 15, i32* %x2)
 ; MAY-NEXT: Alive: <x>
 ; MUST-NEXT: Alive: <>
 
   ret void
 }
 
+define void @multiple_start_end() {
+; CHECK-LABEL: define void @multiple_start_end
+entry:
+; CHECK: entry:
+; CHECK-NEXT: Alive: <>
+  %x = alloca i8
+  call void @llvm.lifetime.start.p0i8(i64 1, i8* %x)
+; CHECK: call void @llvm.lifetime.start.p0i8(i64 1, i8* %x)
+; CHECK-NEXT: Alive: <x>
+
+  call void @llvm.lifetime.start.p0i8(i64 1, i8* %x)
+; CHECK: call void @llvm.lifetime.start.p0i8(i64 1, i8* %x)
+; CHECK-NEXT: Alive: <x>
+
+  call void @llvm.lifetime.end.p0i8(i64 1, i8* %x)
+; CHECK: call void @llvm.lifetime.end.p0i8(i64 1, i8* %x)
+; CHECK-NEXT: Alive: <>
+
+  call void @llvm.lifetime.end.p0i8(i64 1, i8* %x)
+; CHECK: call void @llvm.lifetime.end.p0i8(i64 1, i8* %x)
+; CHECK-NEXT: Alive: <>
+
+  ret void
+}
+
 declare void @llvm.lifetime.start.p0i8(i64, i8* nocapture)
 declare void @llvm.lifetime.end.p0i8(i64, i8* nocapture)
 declare void @llvm.lifetime.start.p0i32(i64, i32* nocapture)
 declare void @llvm.lifetime.end.p0i32(i64, i32* nocapture)
 declare void @capture8(i8*)
 declare void @capture32(i32*)
 declare void @capture64(i64*)
 declare void @capture100x32([100 x i32]*)