transform: do not lower zero-sized alloc to alloca

The LLVM CoroFrame pass appears to be tripping over this zero-sized
alloca. Therefore, do what the runtime would do: return a pointer to
runtime.zeroSizedAlloc. Or just don't deal with this case. But don't
emit a zero sized alloca to avoid this LLVM bug.

More information: https://bugs.llvm.org/show_bug.cgi?id=49916

transform/allocs.go

@@ -43,6 +43,18 @@ func OptimizeAllocs(mod llvm.Module) {
 			continue
 		}
 
+		if size == 0 {
+			// If the size is 0, the pointer is allowed to alias other
+			// zero-sized pointers. Use the pointer to the global that would
+			// also be returned by runtime.alloc.
+			zeroSizedAlloc := mod.NamedGlobal("runtime.zeroSizedAlloc")
+			if !zeroSizedAlloc.IsNil() {
+				heapalloc.ReplaceAllUsesWith(zeroSizedAlloc)
+				heapalloc.EraseFromParentAsInstruction()
+			}
+			continue
+		}
+
 		// In general the pattern is:
 		//     %0 = call i8* @runtime.alloc(i32 %size)
 		//     %1 = bitcast i8* %0 to type*

transform/testdata/allocs.ll

@@ -1,6 +1,8 @@
 target datalayout = "e-m:e-p:32:32-i64:64-v128:64:128-a:0:32-n32-S64"
 target triple = "armv7m-none-eabi"
 
+@runtime.zeroSizedAlloc = internal global i8 0, align 1
+
 declare nonnull i8* @runtime.alloc(i32)
 
 ; Test allocating a single int (i32) that should be allocated on the stack.
@@ -68,6 +70,14 @@ end:
   ret void
 }
 
+; Test a zero-sized allocation.
+define void @testZeroSizedAlloc() {
+  %1 = call i8* @runtime.alloc(i32 0)
+  %2 = bitcast i8* %1 to i32*
+  %3 = call i32* @noescapeIntPtr(i32* %2)
+  ret void
+}
+
 declare i32* @escapeIntPtr(i32*)
 
 declare i32* @noescapeIntPtr(i32* nocapture)

transform/testdata/allocs.out.ll

@@ -1,6 +1,8 @@
 target datalayout = "e-m:e-p:32:32-i64:64-v128:64:128-a:0:32-n32-S64"
 target triple = "armv7m-none-eabi"
 
+@runtime.zeroSizedAlloc = internal global i8 0, align 1
+
 declare nonnull i8* @runtime.alloc(i32)
 
 define void @testInt() {
@@ -66,6 +68,12 @@ end:                                              ; preds = %loop
   ret void
 }
 
+define void @testZeroSizedAlloc() {
+  %1 = bitcast i8* @runtime.zeroSizedAlloc to i32*
+  %2 = call i32* @noescapeIntPtr(i32* %1)
+  ret void
+}
+
 declare i32* @escapeIntPtr(i32*)
 
 declare i32* @noescapeIntPtr(i32* nocapture)