[PATCH,V2 1/3] bpf: Add new -mcore option for BPF CO-RE

Indu Bhagat indu.bhagat@oracle.com
Thu Aug 5 00:50:26 GMT 2021


-mcore in the BPF backend enables code generation for the CO-RE usecase. LTO is
disabled for CO-RE compilations.

gcc/ChangeLog:

	* config/bpf/bpf.c (bpf_option_override): For BPF backend, disable LTO
	support when compiling for CO-RE.
	* config/bpf/bpf.opt: Add new command line option -mcore.

gcc/testsuite/ChangeLog:

	* gcc.target/bpf/core-lto-1.c: New test.
---
 gcc/config/bpf/bpf.c                      | 15 +++++++++++++++
 gcc/config/bpf/bpf.opt                    |  4 ++++
 gcc/testsuite/gcc.target/bpf/core-lto-1.c |  9 +++++++++
 3 files changed, 28 insertions(+)
 create mode 100644 gcc/testsuite/gcc.target/bpf/core-lto-1.c

diff --git a/gcc/config/bpf/bpf.c b/gcc/config/bpf/bpf.c
index e635f9e..028013e 100644
--- a/gcc/config/bpf/bpf.c
+++ b/gcc/config/bpf/bpf.c
@@ -158,6 +158,21 @@ bpf_option_override (void)
 {
   /* Set the initializer for the per-function status structure.  */
   init_machine_status = bpf_init_machine_status;
+
+  /* To support the portability needs of BPF CO-RE approach, BTF debug
+     information includes the BPF CO-RE relocations.  The information
+     necessary for these relocations is added to the CTF container by the
+     BPF backend.  Enabling LTO poses challenges in the generation of the BPF
+     CO-RE relocations because if LTO is in effect, they need to be
+     generated late in the LTO link phase.  This in turn means the compiler
+     needs to provide means to combine the early and late BTF debug info,
+     similar to DWARF debug info.
+
+     In any case, in absence of linker support for BTF sections at this time,
+     it is acceptable to simply disallow LTO for BPF CO-RE compilations.  */
+
+  if (flag_lto && TARGET_BPF_CORE)
+    error ("BPF CO-RE does not support LTO");
 }
 
 #undef TARGET_OPTION_OVERRIDE
diff --git a/gcc/config/bpf/bpf.opt b/gcc/config/bpf/bpf.opt
index 916b53c..e8926f5 100644
--- a/gcc/config/bpf/bpf.opt
+++ b/gcc/config/bpf/bpf.opt
@@ -127,3 +127,7 @@ Generate little-endian eBPF.
 mframe-limit=
 Target Joined RejectNegative UInteger IntegerRange(0, 32767) Var(bpf_frame_limit) Init(512)
 Set a hard limit for the size of each stack frame, in bytes.
+
+mcore
+Target Mask(BPF_CORE)
+Generate all necessary information for BPF Compile Once - Run Everywhere.
diff --git a/gcc/testsuite/gcc.target/bpf/core-lto-1.c b/gcc/testsuite/gcc.target/bpf/core-lto-1.c
new file mode 100644
index 0000000..a90dc5b
--- /dev/null
+++ b/gcc/testsuite/gcc.target/bpf/core-lto-1.c
@@ -0,0 +1,9 @@
+/* Test -mcore with -flto.
+  
+   -mcore is used to generate information for BPF CO-RE usecase. To support
+   the generataion of the .BTF and .BTF.ext sections in GCC, -flto is disabled
+   with -mcore.  */
+
+/* { dg-do compile } */
+/* { dg-error "BPF CO-RE does not support LTO" "" { target bpf-*-* } 0 } */
+/* { dg-options "-gbtf -mcore -flto" } */
-- 
1.8.3.1



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