From c9adc60d730812c1cb09bcce9bf7b4ace91883c3 Mon Sep 17 00:00:00 2001
From: Sintendo <bram.speeckaert@gmail.com>
Date: Sat, 27 Feb 2021 20:51:38 +0100
Subject: [PATCH] Jit64: divwx - Special case dividend == 0

Zero divided by any number is still zero. For whatever reason, this case
shows up frequently too.

Before:
B8 00 00 00 00       mov         eax,0
85 F6                test        esi,esi
74 0C                je          overflow
3D 00 00 00 80       cmp         eax,80000000h
75 0C                jne         normal_path
83 FE FF             cmp         esi,0FFFFFFFFh
75 07                jne         normal_path
overflow:
C1 F8 1F             sar         eax,1Fh
8B F8                mov         edi,eax
EB 05                jmp         done
normal_path:
99                   cdq
F7 FE                idiv        eax,esi
8B F8                mov         edi,eax
done:

After:
Nothing!
---
 .../Core/Core/PowerPC/Jit64/Jit_Integer.cpp   | 107 ++++++++++--------
 1 file changed, 62 insertions(+), 45 deletions(-)

diff --git a/Source/Core/Core/PowerPC/Jit64/Jit_Integer.cpp b/Source/Core/Core/PowerPC/Jit64/Jit_Integer.cpp
index fd6feace7d..88e139abb2 100644
--- a/Source/Core/Core/PowerPC/Jit64/Jit_Integer.cpp
+++ b/Source/Core/Core/PowerPC/Jit64/Jit_Integer.cpp
@@ -1345,57 +1345,74 @@ void Jit64::divwx(UGeckoInstruction inst)
     // Constant dividend
     const u32 dividend = gpr.Imm32(a);
 
-    RCX64Reg Rb = gpr.Bind(b, RCMode::Read);
-    RCX64Reg Rd = gpr.Bind(d, RCMode::Write);
-    // no register choice
-    RCX64Reg eax = gpr.Scratch(EAX);
-    RCX64Reg edx = gpr.Scratch(EDX);
-    RegCache::Realize(Rb, Rd, eax, edx);
-
-    // Check for divisor == 0
-    TEST(32, Rb, Rb);
-
-    FixupBranch normal_path;
-
-    if (dividend == 0x80000000)
+    if (dividend == 0)
     {
-      // Divisor is 0, proceed to overflow case
-      const FixupBranch overflow = J_CC(CC_Z);
-      // Otherwise, check for divisor == -1
-      CMP(32, Rb, Imm32(0xFFFFFFFF));
-      normal_path = J_CC(CC_NE);
+      if (inst.OE)
+      {
+        RCOpArg Rb = gpr.Use(b, RCMode::Read);
+        RegCache::Realize(Rb);
 
-      SetJumpTarget(overflow);
+        CMP_or_TEST(32, Rb, Imm32(0));
+        GenerateOverflow(CC_NZ);
+      }
+
+      // Zero divided by anything is always zero
+      gpr.SetImmediate32(d, 0);
     }
     else
     {
-      // Divisor is not 0, take normal path
-      normal_path = J_CC(CC_NZ);
-      // Otherwise, proceed to overflow case
+      RCX64Reg Rb = gpr.Bind(b, RCMode::Read);
+      RCX64Reg Rd = gpr.Bind(d, RCMode::Write);
+      // no register choice
+      RCX64Reg eax = gpr.Scratch(EAX);
+      RCX64Reg edx = gpr.Scratch(EDX);
+      RegCache::Realize(Rb, Rd, eax, edx);
+
+      // Check for divisor == 0
+      TEST(32, Rb, Rb);
+
+      FixupBranch normal_path;
+
+      if (dividend == 0x80000000)
+      {
+        // Divisor is 0, proceed to overflow case
+        const FixupBranch overflow = J_CC(CC_Z);
+        // Otherwise, check for divisor == -1
+        CMP(32, Rb, Imm32(0xFFFFFFFF));
+        normal_path = J_CC(CC_NE);
+
+        SetJumpTarget(overflow);
+      }
+      else
+      {
+        // Divisor is not 0, take normal path
+        normal_path = J_CC(CC_NZ);
+        // Otherwise, proceed to overflow case
+      }
+
+      // Set Rd to all ones or all zeroes
+      if (dividend & 0x80000000)
+        MOV(32, Rd, Imm32(0xFFFFFFFF));
+      else
+        XOR(32, Rd, Rd);
+
+      if (inst.OE)
+        GenerateConstantOverflow(true);
+
+      const FixupBranch done = J();
+
+      SetJumpTarget(normal_path);
+
+      MOV(32, eax, Imm32(dividend));
+      CDQ();
+      IDIV(32, Rb);
+      MOV(32, Rd, eax);
+
+      if (inst.OE)
+        GenerateConstantOverflow(false);
+
+      SetJumpTarget(done);
     }
-
-    // Set Rd to all ones or all zeroes
-    if (dividend & 0x80000000)
-      MOV(32, Rd, Imm32(0xFFFFFFFF));
-    else
-      XOR(32, Rd, Rd);
-
-    if (inst.OE)
-      GenerateConstantOverflow(true);
-
-    const FixupBranch done = J();
-
-    SetJumpTarget(normal_path);
-
-    MOV(32, eax, Imm32(dividend));
-    CDQ();
-    IDIV(32, Rb);
-    MOV(32, Rd, eax);
-
-    if (inst.OE)
-      GenerateConstantOverflow(false);
-
-    SetJumpTarget(done);
   }
   else
   {