[WebAssembly] Fix FastISel address calculation bug

[tlively]

Summary

Fixes PR47040, in which an assertion was improperly triggered during
FastISel's address computation. The issue was that an Address set to
be relative to the FrameIndex with offset zero was incorrectly
considered to have an unset base. When the left hand side of an add
set the Address to be 0 off the FrameIndex, the right side would not
detect that the Address base had already been set and could try to set
the Address to be relative to a register instead, triggering an
assertion.

This patch fixes the issue by explicitly tracking whether an Address
has been set rather than interpreting an offset of zero to mean the
Address has not been set.

[aheejin]

llvm/test/CodeGen/WebAssembly/fast-isel-pr7040.ll

3 ↗

(On Diff #284113)

The same typo for CL/commit message and the test file name

[sbc100]

"The issue was that an Address set to
be relative to the FrameIndex with offset zero was incorrectly
considered to have an unset base. "

From the patch I can't see how offset (which I assume means the Offset member) comes into play at all. It looks like the fix applies to when either the FI or the Ref itself is zero. Or should I read FrameIndex with offset zero to mean FrameIndex of zero ?

[tlively]

In D85581#2204725, @sbc100 wrote:

"The issue was that an Address set to
be relative to the FrameIndex with offset zero was incorrectly
considered to have an unset base. "

From the patch I can't see how offset (which I assume means the Offset member) comes into play at all. It looks like the fix applies to when either the FI or the Ref itself is zero. Or should I read FrameIndex with offset zero to mean FrameIndex of zero ?

Oh you're right. It's not the Offset but the FrameIndex itself. I'll fix the commit message and comment.

llvm/lib/Target/WebAssembly/WebAssemblyFastISel.cpp

class WebAssemblyFastISel final : public FastISel {
 
   private:
     BaseKind Kind = RegBase;
     union {
       unsigned Reg;
       int FI;
     } Base;
 
+    // Whether the base has been determined yet
+    bool IsBaseSet = false;
+
     int64_t Offset = 0;
 
     const GlobalValue *GV = nullptr;
 
   public:
     // Innocuous defaults for our address.
     Address() { Base.Reg = 0; }
     void setKind(BaseKind K) {
       assert(!isSet() && "Can't change kind with non-zero base");
       Kind = K;
     }
     BaseKind getKind() const { return Kind; }
     bool isRegBase() const { return Kind == RegBase; }
     bool isFIBase() const { return Kind == FrameIndexBase; }
     void setReg(unsigned Reg) {
       assert(isRegBase() && "Invalid base register access!");
-      assert(Base.Reg == 0 && "Overwriting non-zero register");
+      assert(!IsBaseSet && "Base cannot be reset");
       Base.Reg = Reg;
+      IsBaseSet = true;
     }
     unsigned getReg() const {
       assert(isRegBase() && "Invalid base register access!");
       return Base.Reg;
     }
     void setFI(unsigned FI) {
       assert(isFIBase() && "Invalid base frame index access!");
-      assert(Base.FI == 0 && "Overwriting non-zero frame index");
+      assert(!IsBaseSet && "Base cannot be reset");
       Base.FI = FI;
+      IsBaseSet = true;
     }
     unsigned getFI() const {
       assert(isFIBase() && "Invalid base frame index access!");
       return Base.FI;
     }
 
     void setOffset(int64_t NewOffset) {
       assert(NewOffset >= 0 && "Offsets must be non-negative");
       Offset = NewOffset;
     }
     int64_t getOffset() const { return Offset; }
     void setGlobalValue(const GlobalValue *G) { GV = G; }
     const GlobalValue *getGlobalValue() const { return GV; }
-    bool isSet() const {
-      if (isRegBase()) {
-        return Base.Reg != 0;
-      } else {
-        return Base.FI != 0;
-      }
-    }
+    bool isSet() const { return IsBaseSet; }
   };
 
   /// Keep a pointer to the WebAssemblySubtarget around so that we can make the
   /// right decision when generating code for different targets.
   const WebAssemblySubtarget *Subtarget;
   LLVMContext *Context;
 
 private:

llvm/test/CodeGen/WebAssembly/fast-isel-pr47040.ll

+; RUN: llc < %s -fast-isel -fast-isel-abort=1 -verify-machineinstrs
+
+; Regression test for PR47040, in which an assertion was improperly
+; triggered during FastISel's address computation. The issue was that
+; an `Address` set to be relative to FrameIndex zero was incorrectly
+; considered to have an unset base. When the left hand side of an add
+; set the Address to have a FrameIndex base of 0, the right side would
+; not detect that the Address base had already been set and could try
+; to set the Address to be relative to a register instead, triggering
+; an assertion.
+
+target datalayout = "e-m:e-p:32:32-i64:64-n32:64-S128"
+target triple = "wasm32-unknown-unknown"
+
+define i32 @foo() {
+  %stack_addr = alloca i32
+  %stack_i = ptrtoint i32* %stack_addr to i32
+  %added = add i32 %stack_i, undef
+  %added_addr = inttoptr i32 %added to i32*
+  %ret = load i32, i32* %added_addr
+  ret i32 %ret
+}