From cb3a848391434d2fbfbdd2b1f7e38218cd964750 Mon Sep 17 00:00:00 2001
From: Yan Moura <emmetoellder@gmail.com>
Date: Mon, 19 Oct 2026 15:20:12 -0300
Subject: [PATCH] Fix SQRT.S operand selection in the FPU translator

On the R5900, SQRT.S fd, ft takes its source from the ft field (the fs
field is 0 in the encoding). The translator read fs, so a real
"sqrt.s $f2, $f2" (word 0x46020084) became FPU_SQRT_S(f[0]).

Read ft instead. FPU_SQRT_S already ignores the sign of its input, as
the hardware does. Add a code generator test for the encoding.

RSQRT.S had the same kind of bug and was fixed in #272.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HhodgessnwK5k3pipLwFp3
---
 ps2xRecomp/src/lib/fpu_translator.cpp |  3 ++-
 ps2xTest/src/code_generator_tests.cpp | 20 ++++++++++++++++++++
 2 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/ps2xRecomp/src/lib/fpu_translator.cpp b/ps2xRecomp/src/lib/fpu_translator.cpp
index 435db3aa9..00e455c81 100644
--- a/ps2xRecomp/src/lib/fpu_translator.cpp
+++ b/ps2xRecomp/src/lib/fpu_translator.cpp
@@ -58,7 +58,8 @@ namespace ps2recomp
                                    "ctx->f[{}] = FPU_DIV_S(ctx->f[{}], ctx->f[{}]); }}",
                                    ft, fd, fs, ft);
             case COP1_S_SQRT:
-                return fmt::format("ctx->f[{}] = FPU_SQRT_S(ctx->f[{}]);", fd, fs);
+                // SQRT.S fd, ft reads the ft field; fs is 0 in the encoding.
+                return fmt::format("ctx->f[{}] = FPU_SQRT_S(ctx->f[{}]);", fd, ft);
             case COP1_S_ABS:
                 return fmt::format("ctx->f[{}] = FPU_ABS_S(ctx->f[{}]);", fd, fs);
             case COP1_S_MOV:
diff --git a/ps2xTest/src/code_generator_tests.cpp b/ps2xTest/src/code_generator_tests.cpp
index 463b6cff1..6bc6b01f1 100644
--- a/ps2xTest/src/code_generator_tests.cpp
+++ b/ps2xTest/src/code_generator_tests.cpp
@@ -1020,6 +1020,26 @@ void register_code_generator_tests()
             t.IsTrue(ctc1Code.find("ignored") == std::string::npos, "CTC1 FCR31 should not be ignored");
         });
 
+        tc.Run("SQRT.S reads the ft operand", [](TestCase &t) {
+            CodeGenerator gen({}, {});
+
+            // sqrt.s $f2, $f2 (0x46020084): fs field is 0, the source is ft
+            Instruction sqrtS{};
+            sqrtS.raw = 0x46020084;
+            sqrtS.opcode = OPCODE_COP1;
+            sqrtS.rs = COP1_S;
+            sqrtS.rt = 2;
+            sqrtS.rd = 0;
+            sqrtS.sa = 2;
+            sqrtS.function = COP1_S_SQRT;
+
+            std::string sqrtCode = gen.translateInstruction(sqrtS);
+            printGeneratedCode("SQRT.S reads the ft operand", sqrtCode);
+            t.IsTrue(sqrtCode.find("ctx->f[2] = FPU_SQRT_S(ctx->f[2])") != std::string::npos,
+                     "SQRT.S should take the square root of ft");
+            t.IsTrue(sqrtCode.find("ctx->f[0]") == std::string::npos, "SQRT.S must not read fs");
+        });
+
         tc.Run("VU CFC2/CTC2 access VI registers directly", [](TestCase& t)
             {
                 CodeGenerator gen({}, {});