[llvm] [RISCV] Expand PseudoTAIL with t2 instead of t1 for Zicfilp. (PR #89014)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-mc

Author: Yeting Kuo (yetingk)

<details>
<summary>Changes</summary>

PseudoTail should be a software guarded branch in Ziciflp, since its branch target is known in link time. JALR/C.JR/C.JALR with rs1 as t2 is termed a software guarded branch. Such branches do not need to land on a lpad instruction.

ABI Change PR: https://github.com/riscv-non-isa/riscv-asm-manual/pull/93

---
Full diff: https://github.com/llvm/llvm-project/pull/89014.diff


2 Files Affected:

- (modified) llvm/lib/Target/RISCV/MCTargetDesc/RISCVMCCodeEmitter.cpp (+4) 
- (modified) llvm/test/MC/RISCV/tail-call.s (+27) 


``````````diff
diff --git a/llvm/lib/Target/RISCV/MCTargetDesc/RISCVMCCodeEmitter.cpp b/llvm/lib/Target/RISCV/MCTargetDesc/RISCVMCCodeEmitter.cpp
index 5ea386c3c32a3d..0863345b0c6dc6 100644
--- a/llvm/lib/Target/RISCV/MCTargetDesc/RISCVMCCodeEmitter.cpp
+++ b/llvm/lib/Target/RISCV/MCTargetDesc/RISCVMCCodeEmitter.cpp
@@ -126,6 +126,10 @@ void RISCVMCCodeEmitter::expandFunctionCall(const MCInst &MI,
   if (MI.getOpcode() == RISCV::PseudoTAIL) {
     Func = MI.getOperand(0);
     Ra = RISCV::X6;
+    // For Zicfilp, PseudoTAIL should be expanded to a software guarded branch.
+    // It means to use t2(x7) as rs1 of JALR to expand PseudoTAIL.
+    if (STI.hasFeature(RISCV::FeatureStdExtZicfilp))
+      Ra = RISCV::X7;
   } else if (MI.getOpcode() == RISCV::PseudoCALLReg) {
     Func = MI.getOperand(1);
     Ra = MI.getOperand(0).getReg();
diff --git a/llvm/test/MC/RISCV/tail-call.s b/llvm/test/MC/RISCV/tail-call.s
index c94af672edda2a..7c9f28bdfacda4 100644
--- a/llvm/test/MC/RISCV/tail-call.s
+++ b/llvm/test/MC/RISCV/tail-call.s
@@ -12,17 +12,36 @@
 # RUN: llvm-mc -triple riscv64 < %s -show-encoding \
 # RUN:   | FileCheck -check-prefix=FIXUP %s
 
+# RUN: llvm-mc -filetype=obj -triple riscv32 -mattr=+experimental-zicfilp < %s \
+# RUN:   | llvm-objdump -d - | FileCheck --check-prefix=INSTR-ZICFILP %s
+# RUN: llvm-mc -filetype=obj -triple riscv32 -mattr=+experimental-zicfilp < %s \
+# RUN:   | llvm-readobj -r - | FileCheck -check-prefix=RELOC %s
+# RUN: llvm-mc -triple riscv32 -mattr=+experimental-zicfilp < %s -show-encoding \
+# RUN:   | FileCheck -check-prefix=FIXUP %s
+
+# RUN: llvm-mc -filetype=obj -triple riscv64 -mattr=+experimental-zicfilp < %s \
+# RUN:   | llvm-objdump -d - | FileCheck --check-prefix=INSTR-ZICFILP %s
+# RUN: llvm-mc -filetype=obj -triple riscv64 -mattr=+experimental-zicfilp < %s \
+# RUN:   | llvm-readobj -r - | FileCheck -check-prefix=RELOC %s
+# RUN: llvm-mc -triple riscv64 -mattr=+experimental-zicfilp < %s -show-encoding \
+# RUN:   | FileCheck -check-prefix=FIXUP %s
+
 .long foo
 
 tail foo
 # RELOC: R_RISCV_CALL_PLT foo 0x0
 # INSTR: auipc t1, 0
 # INSTR: jr  t1
+# INSTR-ZICFILP: auipc t2, 0
+# INSTR-ZICFILP: jr  t2
 # FIXUP: fixup A - offset: 0, value: foo, kind:
+
 tail bar
 # RELOC: R_RISCV_CALL_PLT bar 0x0
 # INSTR: auipc t1, 0
 # INSTR: jr  t1
+# INSTR-ZICFILP: auipc t2, 0
+# INSTR-ZICFILP: jr  t2
 # FIXUP: fixup A - offset: 0, value: bar, kind:
 
 # Ensure that tail calls to functions whose names coincide with register names
@@ -32,22 +51,30 @@ tail zero
 # RELOC: R_RISCV_CALL_PLT zero 0x0
 # INSTR: auipc t1, 0
 # INSTR: jr  t1
+# INSTR-ZICFILP: auipc t2, 0
+# INSTR-ZICFILP: jr  t2
 # FIXUP: fixup A - offset: 0, value: zero, kind:
 
 tail f1
 # RELOC: R_RISCV_CALL_PLT f1 0x0
 # INSTR: auipc t1, 0
 # INSTR: jr  t1
+# INSTR-ZICFILP: auipc t2, 0
+# INSTR-ZICFILP: jr  t2
 # FIXUP: fixup A - offset: 0, value: f1, kind:
 
 tail ra
 # RELOC: R_RISCV_CALL_PLT ra 0x0
 # INSTR: auipc t1, 0
 # INSTR: jr  t1
+# INSTR-ZICFILP: auipc t2, 0
+# INSTR-ZICFILP: jr  t2
 # FIXUP: fixup A - offset: 0, value: ra, kind:
 
 tail foo at plt
 # RELOC: R_RISCV_CALL_PLT foo 0x0
 # INSTR: auipc t1, 0
 # INSTR: jr  t1
+# INSTR-ZICFILP: auipc t2, 0
+# INSTR-ZICFILP: jr  t2
 # FIXUP: fixup A - offset: 0, value: foo, kind: fixup_riscv_call_plt

``````````

</details>


https://github.com/llvm/llvm-project/pull/89014