tor-browser

commit 8a0ce9ee7a6a3e0479de1d299a00d0f82b9f76aa
parent 177ecde7d631a7201c2f8363cef0373efc6c17fb
Author: André Bargull <andre.bargull@gmail.com>
Date:   Thu, 23 Oct 2025 09:56:10 +0000

Bug 1995496 - Part 1: Use div+mod sequence on risc64. r=spidermonkey-reviewers,iain

From <https://riscv.github.io/riscv-isa-manual/snapshot/unprivileged/#_division_operations>:
> If both the quotient and remainder are required from the same division, the
> recommended code sequence is: `DIV[U] rdq, rs1, rs2; REM[U] rdr, rs1, rs2`
> (`rdq` cannot be the same as `rs1` or `rs2`). Microarchitectures can then
> fuse these into a single divide operation instead of performing two separate
> divides.

Differential Revision: https://phabricator.services.mozilla.com/D269367

Diffstat:
M
js/src/jit/riscv64/MacroAssembler-riscv64.cpp
 | 
30
++++++++++++++++++++++++++++--

1 file changed, 28 insertions(+), 2 deletions(-)
diff --git a/js/src/jit/riscv64/MacroAssembler-riscv64.cpp b/js/src/jit/riscv64/MacroAssembler-riscv64.cpp
@@ -6508,9 +6508,21 @@ void MacroAssemblerRiscv64::ma_div_branch_overflow(Register rd, Register rj,
                                                    Label* overflow) {
   UseScratchRegisterScope temps(this);
   Register scratch = temps.Acquire();
+
+  Register dest = rd;
+  if (rd == rj || rd == rk) {
+    dest = temps.Acquire();
+  }
+
+  // The recommended code sequence to obtain both the quotient and remainder
+  // is div[u] followed by mod[u].
+  ma_div32(dest, rj, rk);
   ma_mod32(scratch, rj, rk);
   ma_b(scratch, scratch, overflow, Assembler::NonZero);
-  divw(rd, rj, rk);
+
+  if (dest != rd) {
+    mv(rd, dest);
+  }
 }
 
 void MacroAssemblerRiscv64::ma_div_branch_overflow(Register rd, Register rj,
@@ -6518,7 +6530,21 @@ void MacroAssemblerRiscv64::ma_div_branch_overflow(Register rd, Register rj,
   UseScratchRegisterScope temps(this);
   Register scratch = temps.Acquire();
   ma_li(scratch, imm);
-  ma_div_branch_overflow(rd, rj, scratch, overflow);
+
+  Register dest = rd;
+  if (rd == rj) {
+    dest = temps.Acquire();
+  }
+
+  // The recommended code sequence to obtain both the quotient and remainder
+  // is div[u] followed by mod[u].
+  ma_div32(dest, rj, scratch);
+  ma_mod32(scratch, rj, scratch);
+  ma_b(scratch, scratch, overflow, Assembler::NonZero);
+
+  if (dest != rd) {
+    mv(rd, dest);
+  }
 }
 
 void MacroAssemblerRiscv64::ma_mod_mask(Register src, Register dest,