[X86] Fix a nullptr defrence in X86Subtarget::classifyLocalReference when compiling with -mcmodel=medium -fpic and using a constant pool

[craig.topper]

Summary

LowerConstantPool passes a nullptr into classifyLocalReference. The medium code model handling for PIC will try to deference it using isa. This patch switches to isa_and_nonnull.

[echristo]

Accepting, couple of options as to some other directions as well if you want to go there.

llvm/lib/Target/X86/X86Subtarget.cpp

91–93

Seems reasonable to either comment your commit message here or try to fix the nullptr into classifyLocalReference and assert nonnull?

[craig.topper]

llvm/lib/Target/X86/X86Subtarget.cpp

91–93

Looks like there are several places that handle constants to that pass null, fastisel, globalisel and regular isel. Also jump tables.

This function takes a GlobalValue and constants and jump table don't exist as GlobalValues do they? So I'm not sure how to fix the nullptr input.

llvm/lib/Target/X86/X86Subtarget.cpp

if (isTargetELF()) {
         return X86II::MO_NO_FLAG;
 
       // The large PIC code model uses GOTOFF.
       case CodeModel::Large:
         return X86II::MO_GOTOFF;
 
       // Medium is a hybrid: RIP-rel for code, GOTOFF for DSO local data.
       case CodeModel::Medium:
-        if (isa<Function>(GV))
+        // Constant pool and jump table handling pass a nullptr to this
+        // function so we need to use isa_and_nonnull.
+        if (isa_and_nonnull<Function>(GV))
           return X86II::MO_NO_FLAG; // All code is RIP-relative
         return X86II::MO_GOTOFF;    // Local symbols use GOTOFF.
       }
       llvm_unreachable("invalid code model");
     }
 
     // Otherwise, this is either a RIP-relative reference or a 64-bit movabsq,
     // both of which use MO_NO_FLAG.

llvm/test/CodeGen/X86/code-model-elf.ll

 ; LARGE-PIC-NEXT:    movq    thread_data@GOTTPOFF(%rip), %rax
 ; LARGE-PIC-NEXT:    movl    %fs:(%rax), %eax
 ; LARGE-PIC-NEXT:    retq
 ;
   %1 = load i32, i32* @thread_data, align 4
   ret i32 %1
 }
 
+define dso_local float @load_constant_pool(float %x) #0 {
+; SMALL-STATIC-LABEL: load_constant_pool:
+; SMALL-STATIC:       # %bb.0:
+; SMALL-STATIC-NEXT:    addss {{\.LCPI.*}}(%rip), %xmm0
+; SMALL-STATIC-NEXT:    retq
+;
+; MEDIUM-STATIC-LABEL: load_constant_pool:
+; MEDIUM-STATIC:       # %bb.0:
+; MEDIUM-STATIC-NEXT:    movabsq ${{\.LCPI.*}}, %rax
+; MEDIUM-STATIC-NEXT:    addss (%rax), %xmm0
+; MEDIUM-STATIC-NEXT:    retq
+;
+; LARGE-STATIC-LABEL: load_constant_pool:
+; LARGE-STATIC:       # %bb.0:
+; LARGE-STATIC-NEXT:    movabsq ${{\.LCPI.*}}, %rax
+; LARGE-STATIC-NEXT:    addss (%rax), %xmm0
+; LARGE-STATIC-NEXT:    retq
+;
+; SMALL-PIC-LABEL: load_constant_pool:
+; SMALL-PIC:       # %bb.0:
+; SMALL-PIC-NEXT:    addss {{\.LCPI.*}}(%rip), %xmm0
+; SMALL-PIC-NEXT:    retq
+;
+; MEDIUM-PIC-LABEL: load_constant_pool:
+; MEDIUM-PIC:       # %bb.0:
+; MEDIUM-PIC-NEXT:    leaq _GLOBAL_OFFSET_TABLE_(%rip), %rax
+; MEDIUM-PIC-NEXT:    movabsq ${{\.LCPI.*}}@GOTOFF, %rcx
+; MEDIUM-PIC-NEXT:    addss (%rax,%rcx), %xmm0
+; MEDIUM-PIC-NEXT:    retq
+;
+; LARGE-PIC-LABEL: load_constant_pool:
+; LARGE-PIC:       # %bb.0:
+; LARGE-PIC-NEXT:  .L11$pb:
+; LARGE-PIC-NEXT:    leaq .L11$pb(%rip), %rax
+; LARGE-PIC-NEXT:    movabsq $_GLOBAL_OFFSET_TABLE_-.L11$pb, %rcx
+; LARGE-PIC-NEXT:    addq %rax, %rcx
+; LARGE-PIC-NEXT:    movabsq ${{\.LCPI.*}}@GOTOFF, %rax
+; LARGE-PIC-NEXT:    addss (%rcx,%rax), %xmm0
+; LARGE-PIC-NEXT:    retq
+  %a = fadd float %x, 1.0
+  ret float %a
+}
+
 attributes #0 = { noinline nounwind uwtable }
 
 !llvm.module.flags = !{!0, !1, !2}
 !llvm.ident = !{!3}
 
 !0 = !{i32 1, !"wchar_size", i32 4}
 !1 = !{i32 7, !"PIC Level", i32 2}
 !2 = !{i32 7, !"PIE Level", i32 2}
 !3 = !{!"clang version 7.0.0 "}