[X86] FastISel fall back on !absolute_symbol GVs

[vlad.tsyrklevich]

Summary

D25878, which added support for !absolute_symbol for normal X86 ISel,
did not add support for materializing references to absolute symbols for
X86 FastISel. This causes build failures because FastISel generates
PC-relative relocations for absolute symbols. Fall back to normal ISel
for references to !absolute_symbol GVs. Fix for PR38200.

[pcc]

LGTM

llvm/lib/Target/X86/X86FastISel.cpp

if (const GlobalValue *GV = dyn_cast<GlobalValue>(V)) {
     // Can't handle alternate code models yet.
     if (TM.getCodeModel() != CodeModel::Small)
       return false;
 
     // Can't handle TLS yet.
     if (GV->isThreadLocal())
       return false;
 
+    // Can't handle !absolute_symbol references yet.
+    if (GV->isAbsoluteSymbolRef())
+      return false;
+
     // RIP-relative addresses can't have additional register operands, so if
     // we've already folded stuff into the addressing mode, just force the
     // global value into its own register, which we can use as the basereg.
     if (!Subtarget->isPICStyleRIPRel() ||
         (AM.Base.Reg == 0 && AM.IndexReg == 0)) {
       // Okay, we've committed to selecting this global. Set up the address.
       AM.GV = GV;
 

llvm/test/CodeGen/X86/absolute-bit-mask-fastisel.ll

+; RUN: llc < %s | FileCheck %s
+; RUN: llc -relocation-model=pic < %s | FileCheck %s
+
+; Regression test for PR38200
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+@bit_mask8 = external hidden global i8, !absolute_symbol !0
+
+declare void @f()
+
+define void @foo8(i8* %ptr) noinline optnone {
+  %load = load i8, i8* %ptr
+  ; CHECK: movl $bit_mask8, %ecx
+  %and = and i8 %load, ptrtoint (i8* @bit_mask8 to i8)
+  %icmp = icmp eq i8 %and, 0
+  br i1 %icmp, label %t, label %f
+
+t:
+  call void @f()
+  ret void
+
+f:
+  ret void
+}
+
+!0 = !{i64 0, i64 256}