[WebAssembly] Fix SSA rebuilding in SjLj transformation

[aheejin]

Summary

Previously we skipped uses within the same BB as a def when rebuilding
SSA after SjLj transformation. For example, before transformation,

for.cond:
  %0 = phi i32 [ %var, %for.inc ] ...
  %var = ...
  br label %for.inc

for.inc:                               ; preds = %for.cond
  call i32 @setjmp(...)
  br %for.cond

In this BB, %var should be defined in all paths from %for.inc to make %0
valid. In the input it was true; %for.inc's only predecessor was
%for.cond. But after SjLj transformation, it is possible that %for.inc
has other predecessors that are reachable without reaching %for.cond.

entry.split:
  ...
  br i1 %a, label %bb.1, label %for.inc

for.cond:
  %0 = phi i32 [ %var, %for.inc ] ...  ; Not valid!
  %var = ...
  br label %for.inc

for.inc:                               ; preds = %for.cond, %entry.split
  call i32 @setjmp(...)
  ...
  br %for.cond

In this case, we can't use %var in the phi instruction in %for.cond,
because %var is not defined in all paths through %for.inc (If the
control flow is %entry -> %entry.split -> %for.inc -> %for.cond, %var
has not been defined until we reach the phi). But the previous code
excluded users within the same BB, skipping instructions within the same
BB so they are not rewritten properly. User instructions within the same
BB also should be candidates for rewriting if they are _before_ the
original definition.

Fixes PR43097.

[aheejin]

llvm/lib/Target/WebAssembly/WebAssemblyLowerEmscriptenEHSjLj.cpp

640

This is just hoisting, because we were doing the same thing unnecessarily repeatedly in the inner loop. Not related to the bugfix itself.

[dschuff]

llvm/test/CodeGen/WebAssembly/lower-em-sjlj.ll

245

Should this be uncommented or removed?

[aheejin]

llvm/test/CodeGen/WebAssembly/lower-em-sjlj.ll

245

Oh no, after the transformation, for.inc itself will be split into multiple BBs, so the CHECK line below will be in for.inc.split BB.

llvm/lib/Target/WebAssembly/WebAssemblyLowerEmscriptenEHSjLj.cpp

 }
 
 void WebAssemblyLowerEmscriptenEHSjLj::rebuildSSA(Function &F) {
   DominatorTree &DT = getAnalysis<DominatorTreeWrapperPass>(F).getDomTree();
   DT.recalculate(F); // CFG has been changed
   SSAUpdater SSA;
   for (BasicBlock &BB : F) {
     for (Instruction &I : BB) {
+      SSA.Initialize(I.getType(), I.getName());
+      SSA.AddAvailableValue(&BB, &I);
       for (auto UI = I.use_begin(), UE = I.use_end(); UI != UE;) {
         Use &U = *UI;
         ++UI;
-        SSA.Initialize(I.getType(), I.getName());
-        SSA.AddAvailableValue(&BB, &I);
         auto *User = cast<Instruction>(U.getUser());
-        if (User->getParent() == &BB)
-          continue;
-
         if (auto *UserPN = dyn_cast<PHINode>(User))
           if (UserPN->getIncomingBlock(U) == &BB)
             continue;
 
         if (DT.dominates(&I, User))
           continue;
         SSA.RewriteUseAfterInsertions(U);
       }

llvm/test/CodeGen/WebAssembly/lower-em-sjlj.ll

 @buffer = global [1 x %struct.__jmp_buf_tag] zeroinitializer, align 16
 define void @longjmp_only() {
 entry:
   ; CHECK: call void @emscripten_longjmp_jmpbuf
   call void @longjmp(%struct.__jmp_buf_tag* getelementptr inbounds ([1 x %struct.__jmp_buf_tag], [1 x %struct.__jmp_buf_tag]* @buffer, i32 0, i32 0), i32 1) #1
   unreachable
 }
 
+; Tests if SSA rewrite works when a use and its def are within the same BB.
+define void @ssa_rewite_in_same_bb() {
+entry:
+  call void @foo()
+  br label %for.cond
+
+for.cond:                                         ; preds = %for.inc, %entry
+  ; CHECK: %{{.*}} = phi i32 [ %var[[VARNO:.*]], %for.inc.split ]
+  %0 = phi i32 [ %var, %for.inc ], [ undef, %entry ]
+  %var = add i32 0, 0
+  br label %for.inc
+
+for.inc:                                          ; preds = %for.cond
+  %call5 = call i32 @setjmp(%struct.__jmp_buf_tag* undef) #0
+  br label %for.cond
+
+; CHECK: for.inc.split:
+  ; CHECK: %var[[VARNO]] = phi i32 [ %var, %for.inc ]
+}
+
 declare void @foo()
 ; Function Attrs: returns_twice
 declare i32 @setjmp(%struct.__jmp_buf_tag*) #0
 ; Function Attrs: noreturn
 declare void @longjmp(%struct.__jmp_buf_tag*, i32) #1
 declare i32 @__gxx_personality_v0(...)
 declare i8* @__cxa_begin_catch(i8*)
 declare void @__cxa_end_catch()