Jit64: divwx - Optimize constant dividend
When the dividend is known at compile time, we can eliminate some of the branching and precompute the result for the overflow case. Before: B8 54 D3 E6 02 mov eax,2E6D354h 85 FF test edi,edi 74 0C je overflow 3D 00 00 00 80 cmp eax,80000000h 75 0C jne normal_path 83 FF FF cmp edi,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 FF idiv eax,edi 8B F8 mov edi,eax done: After: 85 FF test edi,edi 75 04 jne normal_path 33 FF xor edi,edi EB 0A jmp done normal_path: B8 54 D3 E6 02 mov eax,2E6D354h 99 cdq F7 FF idiv eax,edi 8B F8 mov edi,eax done: Fairly common with constant dividend of zero. Non-zero values occur frequently in Ocarina of Time Master Quest.
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@ -1340,6 +1340,63 @@ void Jit64::divwx(UGeckoInstruction inst)
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GenerateConstantOverflow(false);
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}
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}
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else if (gpr.IsImm(a))
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{
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// Constant dividend
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const u32 dividend = gpr.Imm32(a);
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RCX64Reg Rb = gpr.Bind(b, RCMode::Read);
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RCX64Reg Rd = gpr.Bind(d, RCMode::Write);
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// no register choice
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RCX64Reg eax = gpr.Scratch(EAX);
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RCX64Reg edx = gpr.Scratch(EDX);
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RegCache::Realize(Rb, Rd, eax, edx);
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// Check for divisor == 0
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TEST(32, Rb, Rb);
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FixupBranch normal_path;
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if (dividend == 0x80000000)
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{
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// Divisor is 0, proceed to overflow case
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const FixupBranch overflow = J_CC(CC_Z);
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// Otherwise, check for divisor == -1
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CMP(32, Rb, Imm32(0xFFFFFFFF));
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normal_path = J_CC(CC_NE);
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SetJumpTarget(overflow);
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}
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else
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{
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// Divisor is not 0, take normal path
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normal_path = J_CC(CC_NZ);
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// Otherwise, proceed to overflow case
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}
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// Set Rd to all ones or all zeroes
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if (dividend & 0x80000000)
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MOV(32, Rd, Imm32(0xFFFFFFFF));
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else
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XOR(32, Rd, Rd);
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if (inst.OE)
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GenerateConstantOverflow(true);
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const FixupBranch done = J();
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SetJumpTarget(normal_path);
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MOV(32, eax, Imm32(dividend));
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CDQ();
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IDIV(32, Rb);
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MOV(32, Rd, eax);
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if (inst.OE)
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GenerateConstantOverflow(false);
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SetJumpTarget(done);
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}
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else
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{
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RCOpArg Ra = gpr.Use(a, RCMode::Read);
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