spec: Fix unintentional dead code in branching instructions (#963)

Note: This commit assumes that `jump()` and `jump_halfword()`, which
write to `pc` directly, immediately branch upon being executed. And not
that branching is implicitly deferred to end of the instruction.

Assuming the above, some unditional branch instructions have dead code
following a call to `jump()` or `jump_halfword()`. For instance,
`qc.e.jal` would not write the return address to `ra`.

Signed-off-by: Anton Johansson <[email protected]>
Co-authored-by: Anton Johansson <[email protected]>
Co-authored-by: ayosher <[email protected]>

Author: 3 people

spec/custom/isa/qc_iu/inst/Xqci/qc.e.jal.yaml

@@ -31,5 +31,5 @@ access:
   vu: always
 operation(): |
   XReg return_addr = $pc + 6;
-  jump_halfword($pc + imm);
   X[1] = return_addr;
+  jump_halfword($pc + imm);

spec/std/isa/inst/C/c.jal.yaml

@@ -35,5 +35,5 @@ operation(): |
 
   XReg return_addr = $pc + 2;
 
-  jump_halfword($pc + $signed(imm));
   X[1] = return_addr;
+  jump_halfword($pc + $signed(imm));

spec/std/isa/inst/C/c.jalr.yaml

@@ -30,8 +30,9 @@ operation(): |
   if (implemented?(ExtensionName::C) && (CSR[misa].C == 1'b0)) {
     raise(ExceptionCode::IllegalInstruction, mode(), $encoding);
   }
+  XReg addr = X[xs1];
   XReg returnaddr;
   returnaddr = $pc + 2;
 
-  jump(X[xs1]);
   X[1] = returnaddr;
+  jump(addr);

spec/std/isa/inst/I/jal.yaml

@@ -34,8 +34,8 @@ pseudoinstructions:
 operation(): |
   XReg return_addr = $pc + 4;
 
-  jump_halfword($pc + $signed(imm));
   X[xd] = return_addr;
+  jump_halfword($pc + $signed(imm));
 
 # SPDX-SnippetBegin
 # SPDX-FileCopyrightText: 2017-2025 Contributors to the RISCV Sail Model <https://github.com/riscv/sail-riscv/blob/master/LICENCE>

spec/std/isa/inst/I/jalr.yaml

@@ -34,11 +34,12 @@ pseudoinstructions:
   - when: (rd == 0 && xs1 == x1 && imm == 0)
     to: ret
 operation(): |
+  XReg addr = (X[xs1] + $signed(imm)) & ~MXLEN'1;
   XReg returnaddr;
   returnaddr = $pc + 4;
 
-  jump((X[xs1] + $signed(imm)) & ~MXLEN'1);
   X[xd] = returnaddr;
+  jump(addr);
 
 # SPDX-SnippetBegin
 # SPDX-FileCopyrightText: 2017-2025 Contributors to the RISCV Sail Model <https://github.com/riscv/sail-riscv/blob/master/LICENCE>