[llvm] [RISCV] Force relocations if initial MCSubtargetInfo contains FeatureRelax (PR #77436)

[Fangrui Song via llvm-commits]

https://github.com/MaskRay created https://github.com/llvm/llvm-project/pull/77436

Regarding
```
.option norelax
j label
.option relax
// relaxable instructions
label:
```

The J instruction needs a relocation to ensure the target is correct
after linker relaxation. This is related a limitation in the assembler:
RISCVAsmBackend::shouldForceRelocation decides upfront whether a
relocation is needed, instead of checking more information (whether
there are relaxable fragments in between).

Despite the limitation, `j label` produces a relocation in direct object
emission mode, but was broken by #73721 due to the shouldForceRelocation
limitation.

Add a workaround to RISCVTargetELFStreamer to emulate the previous
behavior.

Link: https://github.com/ClangBuiltLinux/linux/issues/1965


>From d91936f761da3efc61086e693b48e26b1926cf6b Mon Sep 17 00:00:00 2001
From: Fangrui Song <i at maskray.me>
Date: Tue, 9 Jan 2024 01:24:36 -0800
Subject: [PATCH] [RISCV] Force relocations if initial MCSubtargetInfo contains
 FeatureRelax

Regarding
```
.option norelax
j label
.option relax
// relaxable instructions
label:
```

The J instruction needs a relocation to ensure the target is correct
after linker relaxation. This is related a limitation in the assembler:
RISCVAsmBackend::shouldForceRelocation decides upfront whether a
relocation is needed, instead of checking more information (whether
there are relaxable fragments in between).

Despite the limitation, `j label` produces a relocation in direct object
emission mode, but was broken by #73721 due to the shouldForceRelocation
limitation.

Add a workaround to RISCVTargetELFStreamer to emulate the previous
behavior.

Link: https://github.com/ClangBuiltLinux/linux/issues/1965
---
 .../RISCV/MCTargetDesc/RISCVELFStreamer.cpp   |  7 +++++
 .../CodeGen/RISCV/option-relax-relocation.ll  | 31 +++++++++++++++++++
 2 files changed, 38 insertions(+)
 create mode 100644 llvm/test/CodeGen/RISCV/option-relax-relocation.ll

diff --git a/llvm/lib/Target/RISCV/MCTargetDesc/RISCVELFStreamer.cpp b/llvm/lib/Target/RISCV/MCTargetDesc/RISCVELFStreamer.cpp
index 9db5148208b3ec..961b8f0afe2254 100644
--- a/llvm/lib/Target/RISCV/MCTargetDesc/RISCVELFStreamer.cpp
+++ b/llvm/lib/Target/RISCV/MCTargetDesc/RISCVELFStreamer.cpp
@@ -37,6 +37,13 @@ RISCVTargetELFStreamer::RISCVTargetELFStreamer(MCStreamer &S,
   auto &MAB = static_cast<RISCVAsmBackend &>(MCA.getBackend());
   setTargetABI(RISCVABI::computeTargetABI(STI.getTargetTriple(), Features,
                                           MAB.getTargetOptions().getABIName()));
+  // `j label` in `.option norelax; j label; .option relax; ...; label:` needs a
+  // relocation to ensure the jump target is correct after linking. This is due
+  // to a limitation that shouldForceRelocation has to make the decision upfront
+  // without knowing a possibly future .option relax. When RISCVAsmParser is used,
+  // its ParseInstruction may call setForceRelocs as well.
+  if (STI.hasFeature(RISCV::FeatureRelax))
+    static_cast<RISCVAsmBackend &>(MAB).setForceRelocs();
 }
 
 RISCVELFStreamer &RISCVTargetELFStreamer::getStreamer() {
diff --git a/llvm/test/CodeGen/RISCV/option-relax-relocation.ll b/llvm/test/CodeGen/RISCV/option-relax-relocation.ll
new file mode 100644
index 00000000000000..f9335f196e10ae
--- /dev/null
+++ b/llvm/test/CodeGen/RISCV/option-relax-relocation.ll
@@ -0,0 +1,31 @@
+;; With +relax, J below needs a relocation to ensure the target is correct
+;; after linker relaxation.
+
+; RUN: llc -mtriple=riscv64 -mattr=-relax -filetype=obj < %s \
+; RUN:     | llvm-objdump -d -r - | FileCheck %s
+; RUN: llc -mtriple=riscv64 -mattr=+relax -filetype=obj < %s \
+; RUN:     | llvm-objdump -d -r - | FileCheck %s --check-prefixes=CHECK,RELAX
+
+; CHECK:        j       {{.*}}
+; RELAX-NEXT:           R_RISCV_JAL  {{.*}}
+; CHECK-NEXT:   auipc   ra, 0x0
+; CHECK-NEXT:           R_RISCV_CALL_PLT     f
+; RELAX-NEXT:           R_RISCV_RELAX        *ABS*
+; CHECK-NEXT:   jalr    ra
+
+define dso_local noundef signext i32 @main() local_unnamed_addr #0 {
+entry:
+  callbr void asm sideeffect ".option push\0A.option norvc\0A.option norelax\0Aj $0\0A.option pop\0A", "!i"() #2
+          to label %asm.fallthrough [label %label]
+
+asm.fallthrough:                                  ; preds = %entry
+  tail call void @f()
+  br label %label
+
+label:                                            ; preds = %asm.fallthrough, %entry
+  ret i32 0
+}
+
+declare void @f()
+
+attributes #0 = { nounwind "target-features"="-c,+relax" }