[AArch64] Add workaround for Cortex-A53 erratum (835769)

[bsmith]

Summary

Some early revisions of the Cortex-A53 have an erratum (835769) whereby it is
possible for a 64-bit multiply-accumulate instruction in AArch64 state to
generate an incorrect result. The details are quite complex and hard to
determine statically, since branches in the code may exist in some
circumstances, but all cases end with a memory (load, store, or prefetch)
instruction followed immediately by the multiply-accumulate operation.

The safest work-around for this issue is to make the compiler avoid emitting
multiply-accumulate instructions immediately after memory instructions and the
simplest way to do this is to insert a NOP.

This patch implements clang options to enable this workaround in the backend.

The work-around code generation is not enabled by default.

[rengolin]

Assuming the LLVM part is good, this looks good to me, too. Thanks!

include/clang/Driver/Options.td

1169

That is a horrible name for a flag, but I don't have a better one. :)

test/Driver/aarch64-fix-cortex-a53-835769-cg.c

18

It's amazing how this works out of the box... :)

[weimingz]

Should we enable it by default if -mcpu=cortex-a53 ?

[bsmith]

Committed as 219604, thanks!

include/clang/Driver/Options.td

 def mgeneral_regs_only : Flag<["-"], "mgeneral-regs-only">, Group<m_aarch64_Features_Group>,
   HelpText<"Generate code which only uses the general purpose registers (AArch64 only)">;
 
+def mfix_cortex_a53_835769 : Flag<["-"], "mfix-cortex-a53-835769">,
+  Group<m_aarch64_Features_Group>,
+  HelpText<"Workaround Cortex-A53 erratum 835769 (AArch64 only)">;
+def mno_fix_cortex_a53_835769 : Flag<["-"], "mno-fix-cortex-a53-835769">,
+  Group<m_aarch64_Features_Group>,
+  HelpText<"Don't workaround Cortex-A53 erratum 835769 (AArch64 only)">;
+
 def mvsx : Flag<["-"], "mvsx">, Group<m_ppc_Features_Group>;
 def mno_vsx : Flag<["-"], "mno-vsx">, Group<m_ppc_Features_Group>;
 def mfprnd : Flag<["-"], "mfprnd">, Group<m_ppc_Features_Group>;

lib/Driver/Tools.cpp

       CmdArgs.push_back("-aarch64-no-strict-align");
   }
 
+  if (Arg *A = Args.getLastArg(options::OPT_mfix_cortex_a53_835769,
+                               options::OPT_mno_fix_cortex_a53_835769)) {
+    CmdArgs.push_back("-backend-option");
+    if (A->getOption().matches(options::OPT_mfix_cortex_a53_835769))
+      CmdArgs.push_back("-aarch64-fix-cortex-a53-835769=1");
+    else
+      CmdArgs.push_back("-aarch64-fix-cortex-a53-835769=0");
+  }
+
   // Setting -mno-global-merge disables the codegen global merge pass. Setting
   // -mglobal-merge has no effect as the pass is enabled by default.
   if (Arg *A = Args.getLastArg(options::OPT_mglobal_merge,

test/Driver/aarch64-fix-cortex-a53-835769-cg.c

+// REQUIRES: aarch64-registered-target
+// RUN: %clang -O3 -target aarch64-linux-eabi %s -S -o- \
+// RUN:   | FileCheck --check-prefix=CHECK-NO --check-prefix=CHECK %s
+// RUN: %clang -O3 -target aarch64-linux-eabi -mfix-cortex-a53-835769 %s -S -o- 2>&1 \
+// RUN:   | FileCheck --check-prefix=CHECK-YES --check-prefix=CHECK %s
+// RUN: %clang -O3 -target aarch64-linux-eabi -mno-fix-cortex-a53-835769 %s -S -o- 2>&1 \
+// RUN:   | FileCheck --check-prefix=CHECK-NO --check-prefix=CHECK %s
+
+#include <stdint.h>
+
+int64_t f_load_madd_64(int64_t a, int64_t b, int64_t *c) {
+    int64_t result = a+b*(*c);
+    return result;
+}
+
+// CHECK: ldr
+// CHECK-YES-NEXT: nop
+// CHECK-NO-NEXT-NOT: nop
+// CHECK-NEXT: madd

test/Driver/aarch64-fix-cortex-a53-835769.c

+// RUN: %clang -target aarch64-linux-eabi %s -### 2>&1 \
+// RUN:   | FileCheck --check-prefix=CHECK-DEF %s
+// RUN: %clang -target aarch64-linux-eabi -mfix-cortex-a53-835769 %s -### 2>&1 \
+// RUN:   | FileCheck --check-prefix=CHECK-YES %s
+// RUN: %clang -target aarch64-linux-eabi -mno-fix-cortex-a53-835769 %s -### 2>&1 \
+// RUN:   | FileCheck --check-prefix=CHECK-NO %s
+// CHECK-DEF-NOT: "-backend-option" "-aarch64-fix-cortex-a53-835769"
+// CHECK-YES: "-backend-option" "-aarch64-fix-cortex-a53-835769=1"
+// CHECK-NO: "-backend-option" "-aarch64-fix-cortex-a53-835769=0"