Falcor fixes (#402)

* Re-define deprecated compile flags

By including these flags in the header file, with a value of zero, we can allow some existing code to compile even after the major changes to the implementation.

* The `SLANG_COMPILE_FLAG_NO_CHECKING` option will effectively be ignored, since checking is always enabled.

* The `SLANG_COMPILE_FLAG_SPLIT_MIXED_TYPES` option will now act as if it is always enabled (and indeed some of the code has been relying on this flag being set always).

* Make subscript operators writable for writable textures

This even had a `TODO` comment saying that we needed to fix it, and now I'm seeing semantic checking failures because we didn't define these and so we find assignment to non l-values.

* Fix definitions of any() and all() intrinsics

These should always return a scalar `bool` value, but they were being defined wrong in two ways:

1. They were using their generic type parameter `T` in the return type
2. They were returning a vector in the vector case, and a matrix in the matrix case.

This change just alters the return type to be `bool` in all cases.

* Fix bug in SSA construction

When eliminating a trivial phi node, it is possible that the phi is still recorded as the "latest" value for a local variable in its block.
When later code queries that value from the block (which can happen whenever another block looks up a variable in its predecessors), it would get the old phi and not the replacement value.

I simply added a loop that checks if the value we look up is a phi that got replaced, and then continues with the replacement value (which might itself be a phi...). A more advanced solution might try to get clever and have the map itself hold `IRUse` values so that we can replace them seamlessly.

* Simplify IR control flow representation

This change gets rid of various special-case operations for conditional and unconditional branches, and instead requires emit logic to recognize when a direct branch is targetting a `break` or `continue` label.

The new approach here isn't perfect, but it seems beter than what we had before, because it can actually work in the presence of control-flow optimizations (including our current critical-edge-splitting step).

* Load from groupshared isn't groupshared

When loading from a `groupshared` variable, the resulting temporary shouldn't have the `groupshared` qualifier on it.
This might eventually need to generalize to a better understanding of storage modifiers in the IR, but I don't really want to deal with that right now.

* Don't emit references to typedefs in output code

Now that we are using the IR for all codegen, we shouldn't be dealing with surface-level things like `typedef` declarations in the output code; just use the type that was being referred to in the first place.

* Fix floating-point literal printing for IR

The IR was calling `emit()` instead of `Emit()` (we really need to normalize our convention here), and was implicitly invoking a default constructor on `String` that takes a `double` (that constructor should really be marked `explicit`), and which doesn't meet our requirements for printing floating-point values.

* Fix error when importing module that doesn't parse

We already added a case to bail out if semantic checking fails, but neglected to add a case if there is an error during parsing of a module to be imported.

Note: this logic doesn't correctly register the module as being loaded (but still in error), so users could see multiple error messages if there are multiple `import`s for the same module.

* Improve error message for overload resolution failure

- Drop debugging info from the candidate printing

- Add cases to print `double` and `half` types properly

* Fixup: switch loopTest to ifElse in expected IR output

1 files changed, 38 insertions, 11 deletions

diff --git a/source/slang/ir-ssa.cpp b/source/slang/ir-ssa.cpp
index 366ecc279..8a2d201dd 100644
--- a/source/slang/ir-ssa.cpp
+++ b/source/slang/ir-ssa.cpp
@@ -28,9 +28,9 @@ struct PhiInfo : RefObject
     // order in which the predecessor blocks get enumerated.
     List<IRUse> operands;
 
-    // Did we end up removing this phi because it was determined
-    // to be trivial?
-    bool wasRemoved = false;
+    // If this phi ended up being removed as trivial, then
+    // this will be the value that we replaced it with.
+    IRValue* replacement = nullptr;
 };
 
 // Information about a basic block that we generate/use
@@ -270,12 +270,16 @@ IRValue* tryRemoveTrivialPhi(
     // of itself) with the unique non-phi value.
     phi->replaceUsesWith(same);
 
-    // We will mark the phi as removed in the `PhiInfo` structure,
-    // so that we can avoid adding it to the block later.
-    phiInfo->wasRemoved = true;
+    // Clear out the operands to the phi, since they won't
+    // actually get used in the program any more.
+    for( auto& u : phiInfo->operands )
+    {
+        u.clear();
+    }
 
-    // TODO: we need a way to record that this parameter
-    // is no longer to be used...
+    // We will record the value that was used to replace this
+    // phi, so that we can easily look it up later.
+    phiInfo->replacement = same;
 
     // TODO: need to recursively consider chances to simplify
     // other phi nodes now.
@@ -476,6 +480,29 @@ IRValue* readVar(
     {
         // Hooray, we found a value to use, and we
         // can proceed without too many complications.
+
+        // Well, let's check for one special case here, which
+        // is when the value we intend to use is a `phi`
+        // node that we ultimately decided to remove.
+        while( val->op == kIROp_Param )
+        {
+            // The value is a parameter, but is it a phi?
+            IRParam* maybePhi = (IRParam*) val;
+            RefPtr<PhiInfo> phiInfo = nullptr;
+            if(!context->phiInfos.TryGetValue(maybePhi, phiInfo))
+                break;
+
+            // Okay, this is indeed a phi we are adding, but
+            // is it one that got replaced?
+            if(!phiInfo->replacement)
+                break;
+
+            // The phi we want to use got replaced, so we
+            // had better use the replacement instead.
+            val = phiInfo->replacement;
+        }
+
+
         return val;
     }
 
@@ -724,9 +751,9 @@ void constructSSA(ConstructSSAContext* context)
 
         for (auto phiInfo : blockInfo->phis)
         {
-            // If we eliminated this phi, then we had better not
-            // include it in the result.
-            if (phiInfo->wasRemoved)
+            // If we replaced this phi with another value,
+            // then we had better not include it in the result.
+            if (phiInfo->replacement)
                 continue;
 
             // We should add the phi as an explicit parameter of