Teach IPSCCP to not propagate return values of naked functions

[davide]

Summary

There can be ASM returning some other value, and we might end up folding the incorrect value.

[Bigcheese]

lgtm.

I wonder if there are any other interprocedural passes which get this wrong.

[davide]

Probably at least IPConstprop (the 'naive version' of SCCP that doesn't build a lattice yada yada yada), but that's not part of the default pipeline. I'll take care of it next.

lib/Transforms/Scalar/SCCP.cpp

   // taken or that are external as overdefined.
   //
   for (Function &F : M) {
     if (F.isDeclaration())
       continue;
 
     // If this is an exact definition of this function, then we can propagate
     // information about its result into callsites of it.
-    if (F.hasExactDefinition())
+    // Don't touch naked functions. They may contain asm returning a
+    // value we don't see, so we may end up interprocedurally propagating
+    // the return value incorrectly.
+    if (F.hasExactDefinition() && !F.hasFnAttribute(Attribute::Naked))
       Solver.AddTrackedFunction(&F);
 
     // If this function only has direct calls that we can see, we can track its
     // arguments and return value aggressively, and can assume it is not called
     // unless we see evidence to the contrary.
     if (F.hasLocalLinkage()) {
       if (AddressIsTaken(&F))
         AddressTakenFunctions.insert(&F);

test/Transforms/IPConstantProp/naked-return.ll

+; RUN: opt -ipsccp %s | FileCheck %s
+
+target datalayout = "e-m:x-p:32:32-i64:64-f80:32-n8:16:32-a:0:32-S32"
+target triple = "i686-pc-windows-msvc19.0.24215"
+
+define i32 @dipsy(i32, i32) local_unnamed_addr #0 {
+BasicBlock0:
+  call void asm "\0D\0Apushl %ebp\0D\0Amovl 8(%esp),%eax\0D\0Amovl 12(%esp), %ebp\0D\0Acalll *%eax\0D\0Apopl %ebp\0D\0Aretl\0D\0A", ""()
+  ret i32 0
+}
+
+define void @tinkywinky(i32, i32, i32) local_unnamed_addr #0 {
+BasicBlock1:
+  call void asm "\0D\0A    movl 12(%esp), %ebp\0D\0A    movl 4(%esp), %eax\0D\0A    movl 8(%esp), %esp\0D\0A    jmpl *%eax\0D\0A", ""()
+  ret void
+}
+
+define void @patatino(i32, i32, i32) local_unnamed_addr #1 {
+bb:
+  %3 = tail call i32 @dipsy(i32 %0, i32 %1) #0
+; Check that we don't accidentally propagate zero.
+; CHECK: @tinkywinky(i32 %3, i32 %2, i32 %1) #0
+  tail call void @tinkywinky(i32 %3, i32 %2, i32 %1) #0
+  ret void
+}
+
+attributes #0 = { naked noinline optnone }
+attributes #1 = { "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" }