XARCH: Remove redudant tests for GT_LT/GT_GE relops. (#61152)

* XARCH: Remove redudant tests for GT_LT/GT_GE relops.

We can now optimize cases such as `(x + y < 0)` or `for (int x = v; x >= 0; x--)`
using the flag tracking logic during the emit stage. Notably, cases that
would generate...

```
add     reg0, reg1
test    reg0, reg0
jge     LABEL
```

now transform to

```
add     reg0, reg1
jns     LABEL
```

This transform is valid for signed GE and signed LT only.

* Add a few asserts related to flag reuse optimizations.

Author: anthonycanino

src/coreclr/jit/codegenlinear.cpp

@@ -2631,6 +2631,16 @@ void CodeGen::genCodeForJumpTrue(GenTreeOp* jtrue)
 
         condition = GenCondition(GenCondition::P);
     }
+
+    if (relop->MarkedForSignJumpOpt())
+    {
+        // If relop was previously marked for a signed jump check optimization because of SF flag
+        // reuse, replace jge/jl with jns/js.
+
+        assert(relop->OperGet() == GT_LT || relop->OperGet() == GT_GE);
+        condition = (relop->OperGet() == GT_LT) ? GenCondition(GenCondition::S) : GenCondition(GenCondition::NS);
+    }
+
 #endif
 
     inst_JCC(condition, compiler->compCurBB->bbJumpDest);

src/coreclr/jit/codegenxarch.cpp

@@ -6227,11 +6227,18 @@ void CodeGen::genCompareInt(GenTree* treeNode)
     assert(genTypeSize(type) <= genTypeSize(TYP_I_IMPL));
     // TYP_UINT and TYP_ULONG should not appear here, only small types can be unsigned
     assert(!varTypeIsUnsigned(type) || varTypeIsSmall(type));
+    // Sign jump optimization should only be set the following check
+    assert((tree->gtFlags & GTF_RELOP_SJUMP_OPT) == 0);
 
     if (canReuseFlags && emit->AreFlagsSetToZeroCmp(op1->GetRegNum(), emitTypeSize(type), tree->OperGet()))
     {
         JITDUMP("Not emitting compare due to flags being already set\n");
     }
+    else if (canReuseFlags && emit->AreFlagsSetForSignJumpOpt(op1->GetRegNum(), emitTypeSize(type), tree))
+    {
+        JITDUMP("Not emitting compare due to sign being already set, follow up instr will transform jump\n");
+        tree->gtFlags |= GTF_RELOP_SJUMP_OPT;
+    }
     else
     {
         emit->emitInsBinary(ins, emitTypeSize(type), op1, op2);

src/coreclr/jit/emitxarch.cpp

@@ -164,6 +164,21 @@ bool emitter::DoesWriteZeroFlag(instruction ins)
     return (CodeGenInterface::instInfo[ins] & Writes_ZF) != 0;
 }
 
+//------------------------------------------------------------------------
+// DoesWriteSignFlag: check if the instruction writes the
+//     SF flag.
+//
+// Arguments:
+//    ins - instruction to test
+//
+// Return Value:
+//    true if instruction writes the SF flag, false otherwise.
+//
+bool emitter::DoesWriteSignFlag(instruction ins)
+{
+    return (CodeGenInterface::instInfo[ins] & Writes_SF) != 0;
+}
+
 //------------------------------------------------------------------------
 // DoesResetOverflowAndCarryFlags: check if the instruction resets the
 //     OF and CF flag to 0.
@@ -338,6 +353,11 @@ bool emitter::AreFlagsSetToZeroCmp(regNumber reg, emitAttr opSize, genTreeOps tr
 {
     assert(reg != REG_NA);
 
+    if (!emitComp->opts.OptimizationEnabled())
+    {
+        return false;
+    }
+
     // Don't look back across IG boundaries (possible control flow)
     if (emitCurIGinsCnt == 0 && ((emitCurIG->igFlags & IGF_EXTEND) == 0))
     {
@@ -393,6 +413,79 @@ bool emitter::AreFlagsSetToZeroCmp(regNumber reg, emitAttr opSize, genTreeOps tr
     return false;
 }
 
+//------------------------------------------------------------------------
+// AreFlagsSetToForSignJumpOpt: checks if the previous instruction set the SF if the tree
+//                              node qualifies for a jg/jle to jns/js optimization
+//
+// Arguments:
+//    reg     - register of interest
+//    opSize  - size of register
+//    relop   - relational tree node
+//
+// Return Value:
+//    true if the tree node qualifies for the jg/jle to jns/js optimization
+//    false if not, or if we can't safely determine
+//
+// Notes:
+//    Currently only looks back one instruction.
+bool emitter::AreFlagsSetForSignJumpOpt(regNumber reg, emitAttr opSize, GenTree* relop)
+{
+    assert(reg != REG_NA);
+
+    if (!emitComp->opts.OptimizationEnabled())
+    {
+        return false;
+    }
+
+    // Don't look back across IG boundaries (possible control flow)
+    if (emitCurIGinsCnt == 0 && ((emitCurIG->igFlags & IGF_EXTEND) == 0))
+    {
+        return false;
+    }
+
+    instrDesc*  id      = emitLastIns;
+    instruction lastIns = id->idIns();
+    insFormat   fmt     = id->idInsFmt();
+
+    // make sure op1 is a reg
+    switch (fmt)
+    {
+        case IF_RWR_CNS:
+        case IF_RRW_CNS:
+        case IF_RRW_SHF:
+        case IF_RWR_RRD:
+        case IF_RRW_RRD:
+        case IF_RWR_MRD:
+        case IF_RWR_SRD:
+        case IF_RRW_SRD:
+        case IF_RWR_ARD:
+        case IF_RRW_ARD:
+        case IF_RWR:
+        case IF_RRD:
+        case IF_RRW:
+            break;
+        default:
+            return false;
+    }
+
+    if (id->idReg1() != reg)
+    {
+        return false;
+    }
+
+    // If we have a GT_GE/GT_LT which generates an jge/jl, and the previous instruction
+    // sets the SF, we can omit a test instruction and check for jns/js.
+    if ((relop->OperGet() == GT_GE || relop->OperGet() == GT_LT) && !GenCondition::FromRelop(relop).IsUnsigned())
+    {
+        if (DoesWriteSignFlag(lastIns) && IsFlagsAlwaysModified(id))
+        {
+            return id->idOpSize() == opSize;
+        }
+    }
+
+    return false;
+}
+
 //------------------------------------------------------------------------
 // IsDstSrcImmAvxInstruction: Checks if the instruction has a "reg, reg/mem, imm" or
 //                            "reg/mem, reg, imm" form for the legacy, VEX, and EVEX

src/coreclr/jit/emitxarch.h

@@ -116,6 +116,7 @@ static bool IsJmpInstruction(instruction ins);
 bool AreUpper32BitsZero(regNumber reg);
 
 bool AreFlagsSetToZeroCmp(regNumber reg, emitAttr opSize, genTreeOps treeOps);
+bool AreFlagsSetForSignJumpOpt(regNumber reg, emitAttr opSize, GenTree* tree);
 
 bool hasRexPrefix(code_t code)
 {
@@ -190,6 +191,7 @@ void SetContains256bitAVX(bool value)
 bool IsDstDstSrcAVXInstruction(instruction ins);
 bool IsDstSrcSrcAVXInstruction(instruction ins);
 bool DoesWriteZeroFlag(instruction ins);
+bool DoesWriteSignFlag(instruction ins);
 bool DoesResetOverflowAndCarryFlags(instruction ins);
 bool IsFlagsAlwaysModified(instrDesc* id);
 

src/coreclr/jit/gentree.h

@@ -531,6 +531,8 @@ enum GenTreeFlags : unsigned int
     GTF_RELOP_QMARK             = 0x20000000, // GT_<relop> -- the node is the condition for ?:
     GTF_RELOP_ZTT               = 0x08000000, // GT_<relop> -- Loop test cloned for converting while-loops into do-while
                                               //               with explicit "loop test" in the header block.
+    GTF_RELOP_SJUMP_OPT         = 0x04000000, // GT_<relop> -- Swap signed jl/jge with js/jns during emitter, reuses flags 
+                                              //               from previous instruction.
 
     GTF_JCMP_EQ                 = 0x80000000, // GTF_JCMP_EQ  -- Branch on equal rather than not equal
     GTF_JCMP_TST                = 0x40000000, // GTF_JCMP_TST -- Use bit test instruction rather than compare against zero instruction
@@ -3028,6 +3030,13 @@ struct GenTreeOp : public GenTreeUnOp
     {
     }
 #endif
+
+    // True if this relop is marked for a transform during the emitter
+    // phase, e.g., jge => jns
+    bool MarkedForSignJumpOpt() const
+    {
+        return (gtFlags & GTF_RELOP_SJUMP_OPT) != 0;
+    }
 };
 
 struct GenTreeVal : public GenTree