x64: support PC-rel symbol references using the GOT when in PIC mode.

cranelift/codegen/src/isa/x64/inst/emit.rs

@@ -2690,6 +2690,30 @@ pub(crate) fn emit(
         }
 
         Inst::LoadExtName { dst, name, offset } => {
+            if info.flags().is_pic() {
+                // Generates: movq symbol@GOTPCREL(%rip), %dst
+                let enc_dst = int_reg_enc(dst.to_reg());
+                sink.put1(0x48 | ((enc_dst >> 3) & 1) << 2);
+                sink.put1(0x8B);
+                sink.put1(0x05 | ((enc_dst & 7) << 3));
+                emit_reloc(sink, state, Reloc::X86GOTPCRel4, name, -4);
+                sink.put4(0);
+                // Offset in the relocation above applies to the address of the *GOT entry*, not
+                // the loaded address; so we emit a separate add or sub instruction if needed.
+                if *offset < 0 {
+                    assert!(*offset >= -i32::MAX as i64);
+                    sink.put1(0x48 | ((enc_dst >> 3) & 1));
+                    sink.put1(0x81);
+                    sink.put1(0xe8 | (enc_dst & 7));
+                    sink.put4((-*offset) as u32);
+                } else if *offset > 0 {
+                    assert!(*offset <= i32::MAX as i64);
+                    sink.put1(0x48 | ((enc_dst >> 3) & 1));
+                    sink.put1(0x81);
+                    sink.put1(0xc0 | (enc_dst & 7));
+                    sink.put4(*offset as u32);
+                }
+            } else {
                 // The full address can be encoded in the register, with a relocation.
                 // Generates: movabsq $name, %dst
                 let enc_dst = int_reg_enc(dst.to_reg());
@@ -2702,6 +2726,7 @@ pub(crate) fn emit(
                     sink.put8(0);
                 }
             }
+        }
 
         Inst::LockCmpxchg { ty, src, dst } => {
             // lock cmpxchg{b,w,l,q} %src, (dst)

cranelift/codegen/src/isa/x64/inst/emit_tests.rs

@@ -2778,6 +2778,46 @@ fn test_x64_emit() {
         "call    *321(%r10,%rdx,4)",
     ));
 
+    // ========================================================
+    // LoadExtName
+    // N.B.: test harness below sets is_pic.
+    insns.push((
+        Inst::LoadExtName {
+            dst: Writable::from_reg(r11),
+            name: Box::new(ExternalName::User {
+                namespace: 0,
+                index: 0,
+            }),
+            offset: 0,
+        },
+        "4C8B1D00000000",
+        "load_ext_name u0:0+0, %r11",
+    ));
+    insns.push((
+        Inst::LoadExtName {
+            dst: Writable::from_reg(r11),
+            name: Box::new(ExternalName::User {
+                namespace: 0,
+                index: 0,
+            }),
+            offset: 0x12345678,
+        },
+        "4C8B1D000000004981C378563412",
+        "load_ext_name u0:0+305419896, %r11",
+    ));
+    insns.push((
+        Inst::LoadExtName {
+            dst: Writable::from_reg(r11),
+            name: Box::new(ExternalName::User {
+                namespace: 0,
+                index: 0,
+            }),
+            offset: -0x12345678,
+        },
+        "4C8B1D000000004981EB78563412",
+        "load_ext_name u0:0+-305419896, %r11",
+    ));
+
     // ========================================================
     // Ret
     insns.push((Inst::ret(), "C3", "ret"));
@@ -3693,7 +3733,9 @@ fn test_x64_emit() {
 
     // ========================================================
     // Actually run the tests!
-    let flags = settings::Flags::new(settings::builder());
+    let mut flag_builder = settings::builder();
+    flag_builder.enable("is_pic").unwrap();
+    let flags = settings::Flags::new(flag_builder);
 
     use crate::settings::Configurable;
     let mut isa_flag_builder = x64::settings::builder();

cranelift/codegen/src/isa/x64/inst/mod.rs

@@ -396,7 +396,10 @@ pub enum Inst {
     /// An instruction that will always trigger the illegal instruction exception.
     Ud2 { trap_code: TrapCode },
 
-    /// Loads an external symbol in a register, with a relocation: movabsq $name, dst
+    /// Loads an external symbol in a register, with a relocation:
+    ///
+    /// movq $name@GOTPCREL(%rip), dst    if PIC is enabled, or
+    /// movabsq $name, dst                otherwise.
     LoadExtName {
         dst: Writable<Reg>,
         name: Box<ExternalName>,
@@ -1692,7 +1695,7 @@ impl PrettyPrint for Inst {
                 dst, name, offset, ..
             } => format!(
                 "{} {}+{}, {}",
-                ljustify("movaps".into()),
+                ljustify("load_ext_name".into()),
                 name,
                 offset,
                 show_ireg_sized(dst.to_reg(), mb_rru, 8),