From 30d512144e694cf63cc78b6fff8c9d52ac89385e Mon Sep 17 00:00:00 2001
From: Andreas Kling <kling@serenityos.org>
Date: Sat, 18 Jul 2020 16:02:51 +0200
Subject: [PATCH] UserspaceEmulator: Implement the BSF and BSF instructions

BSF maps nicely to __builtin_ctz(), but for BSR we have to bust out
some inline assembly to get exactly what we want.
---
 DevTools/UserspaceEmulator/SoftCPU.cpp | 59 ++++++++++++++++++++++++--
 1 file changed, 55 insertions(+), 4 deletions(-)

diff --git a/DevTools/UserspaceEmulator/SoftCPU.cpp b/DevTools/UserspaceEmulator/SoftCPU.cpp
index 73323d322d..0dee78aca7 100644
--- a/DevTools/UserspaceEmulator/SoftCPU.cpp
+++ b/DevTools/UserspaceEmulator/SoftCPU.cpp
@@ -852,10 +852,61 @@ void SoftCPU::AAM(const X86::Instruction&) { TODO(); }
 void SoftCPU::AAS(const X86::Instruction&) { TODO(); }
 void SoftCPU::ARPL(const X86::Instruction&) { TODO(); }
 void SoftCPU::BOUND(const X86::Instruction&) { TODO(); }
-void SoftCPU::BSF_reg16_RM16(const X86::Instruction&) {  }
-void SoftCPU::BSF_reg32_RM32(const X86::Instruction&) { }
-void SoftCPU::BSR_reg16_RM16(const X86::Instruction&) { }
-void SoftCPU::BSR_reg32_RM32(const X86::Instruction&) { }
+
+template<typename T>
+ALWAYS_INLINE static unsigned op_bsf(SoftCPU&, T value)
+{
+    return __builtin_ctz(value);
+}
+
+template<typename T>
+ALWAYS_INLINE static unsigned op_bsr(SoftCPU&, T value)
+{
+    T bit_index = 0;
+    if constexpr (sizeof(T) == 4) {
+        asm volatile("bsrl %%eax, %%edx"
+                     : "=d"(bit_index)
+                     : "a"(value));
+    }
+    if constexpr (sizeof(T) == 2) {
+        asm volatile("bsrw %%ax, %%dx"
+                     : "=d"(bit_index)
+                     : "a"(value));
+    }
+    return bit_index;
+}
+
+void SoftCPU::BSF_reg16_RM16(const X86::Instruction& insn)
+{
+    auto src = insn.modrm().read16(*this, insn);
+    set_zf(!src);
+    if (src)
+        gpr16(insn.reg16()) = op_bsf(*this, src);
+}
+
+void SoftCPU::BSF_reg32_RM32(const X86::Instruction& insn)
+{
+    auto src = insn.modrm().read32(*this, insn);
+    set_zf(!src);
+    if (src)
+        gpr32(insn.reg32()) = op_bsf(*this, insn.modrm().read32(*this, insn));
+}
+
+void SoftCPU::BSR_reg16_RM16(const X86::Instruction& insn)
+{
+    auto src = insn.modrm().read16(*this, insn);
+    set_zf(!src);
+    if (src)
+        gpr16(insn.reg16()) = op_bsr(*this, insn.modrm().read16(*this, insn));
+}
+
+void SoftCPU::BSR_reg32_RM32(const X86::Instruction& insn)
+{
+    auto src = insn.modrm().read32(*this, insn);
+    set_zf(!src);
+    if (src)
+        gpr32(insn.reg32()) = op_bsr(*this, insn.modrm().read32(*this, insn));
+}
 
 void SoftCPU::BSWAP_reg32(const X86::Instruction& insn)
 {