[WebAssembly] Revise the strategy for inline asm.

Previously, an "r" constraint would mean the compiler provides a value
on WebAssembly's operand stack. This was tricky to use properly,
particularly since it isn't possible to declare a new local from within
an inline asm string.

With this patch, "r" provides the value in a WebAssembly local, and the
local index is provided to the inline asm string. This requires inline
asm to use get_local and set_local to read the register. This does
potentially result in larger code size, however inline asm should
hopefully be quite rare in WebAssembly.

This also means that the "m" constraint can no longer be supported, as
WebAssembly has nothing like a "memory operand" that includes an
implicit get_local.

This fixes PR34599 for the wasm32-unknown-unknown-wasm target (though
not for the ELF target).

llvm-svn: 317707

1 files changed, 5 insertions, 6 deletions

diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyAsmPrinter.cpp b/llvm/lib/Target/WebAssembly/WebAssemblyAsmPrinter.cpp
index 211358ad66c..ee60c8f3a7a 100644
--- a/llvm/lib/Target/WebAssembly/WebAssemblyAsmPrinter.cpp
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyAsmPrinter.cpp
@@ -267,12 +267,11 @@ bool WebAssemblyAsmPrinter::PrintAsmMemoryOperand(const MachineInstr *MI,
   if (AsmVariant != 0)
     report_fatal_error("There are no defined alternate asm variants");
 
-  if (!ExtraCode) {
-    // TODO: For now, we just hard-code 0 as the constant offset; teach
-    // SelectInlineAsmMemoryOperand how to do address mode matching.
-    OS << "0(" + regToString(MI->getOperand(OpNo)) + ')';
-    return false;
-  }
+  // The current approach to inline asm is that "r" constraints are expressed
+  // as local indices, rather than values on the operand stack. This simplifies
+  // using "r" as it eliminates the need to push and pop the values in a
+  // particular order, however it also makes it impossible to have an "m"
+  // constraint. So we don't support it.
 
   return AsmPrinter::PrintAsmMemoryOperand(MI, OpNo, AsmVariant, ExtraCode, OS);
 }