[mips] [IAS] Fix expansion of LASym with negative offset.

[tomatabacu]

Summary

If the RHS is a MCConstantExpr and we are subtracting it, we switch the MCBinaryExpr to be the
addition of the negated RHS MCConstantExpr with the unchanged LHS.

[tomatabacu]

Rebased.

[dsanders]

lib/Target/Mips/AsmParser/MipsAsmParser.cpp

2646–2661

At first glance, this doesn't seem like the right thing to do. Could you explain why we can't support 'Sym - Constant'?

I suspect you may be trying to work around problems caused by the broken code on lines 2530-2555. If that's the case then we should fix the root cause instead.

test/MC/Mips/mips-expansions.s

61–76

The expression should be the same for both %hi and %lo. What happens when a carry bit crosses between bit 16 and bit 15?

[tomatabacu]

Rebased (for Sean Bruno's convenience).

[tomatabacu]

Abandoned because evaluateRelocExpr() needs to be rewritten anyway.

lib/Target/Mips/AsmParser/MipsAsmParser.cpp

const MCExpr *MipsAsmParser::evaluateRelocExpr(const MCExpr *Expr,
   if (const MCBinaryExpr *BE = dyn_cast<MCBinaryExpr>(Expr)) {
     MCSymbolRefExpr::VariantKind VK = getVariantKind(RelocStr);
 
     // Try to create target expression.
     if (MipsMCExpr::isSupportedBinaryExpr(VK, BE))
       return MipsMCExpr::create(VK, Expr, getContext());
 
     const MCExpr *LExp = evaluateRelocExpr(BE->getLHS(), RelocStr);
+    // If the RHS is a constant and we are subtracting it, switch the
+    // MCBinaryExpr to be the addition of the negated RHS constant with the
+    // unchanged LHS.
+    if (BE->getOpcode() == MCBinaryExpr::Sub &&
+        isa<MCConstantExpr>(BE->getRHS())) {
+      const MCConstantExpr *OldRExp = cast<MCConstantExpr>(BE->getRHS());
+      const MCExpr *NewRExp =
+          MCConstantExpr::create(-OldRExp->getValue(), getContext());
+
+      const MCExpr *RExp = evaluateRelocExpr(NewRExp, RelocStr);
+      Res = MCBinaryExpr::create(MCBinaryExpr::Add, LExp, RExp, getContext());
+    } else {
       const MCExpr *RExp = evaluateRelocExpr(BE->getRHS(), RelocStr);
       Res = MCBinaryExpr::create(BE->getOpcode(), LExp, RExp, getContext());
+    }
     return Res;
   }
 
   if (const MCUnaryExpr *UN = dyn_cast<MCUnaryExpr>(Expr)) {
     const MCExpr *UnExp = evaluateRelocExpr(UN->getSubExpr(), RelocStr);
     Res = MCUnaryExpr::create(UN->getOpcode(), UnExp, getContext());
     return Res;
   }
   // Just return the original expression.

test/MC/Mips/mips-expansions.s

 # CHECK:                               #   fixup A - offset: 0, value: symbol@ABS_HI, kind: fixup_Mips_HI16
 # CHECK: ori $8, $8, %lo(symbol+2)     # encoding: [0x02'A',A,0x08,0x35]
 # CHECK:                               #   fixup A - offset: 0, value: symbol@ABS_LO, kind: fixup_Mips_LO16
   la $8, symbol+0x11111
 # CHECK: lui $8, %hi(symbol+1)         # encoding: [0x01'A',A,0x08,0x3c]
 # CHECK:                               #   fixup A - offset: 0, value: symbol@ABS_HI, kind: fixup_Mips_HI16
 # CHECK: ori $8, $8, %lo(symbol+4369)  # encoding: [0x11'A',0x11'A',0x08,0x35]
 # CHECK:                               #   fixup A - offset: 0, value: symbol@ABS_LO, kind: fixup_Mips_LO16
+  la $8, symbol-1
+# CHECK: lui $8, %hi(symbol)           # encoding: [A,A,0x08,0x3c]
+# CHECK:                               #   fixup A - offset: 0, value: symbol@ABS_HI, kind: fixup_Mips_HI16
+# CHECK: ori $8, $8, %lo(symbol-1)     # encoding: [0xff'A',0xff'A',0x08,0x35]
+# CHECK:                               #   fixup A - offset: 0, value: symbol@ABS_LO, kind: fixup_Mips_LO16
+  la $8, symbol-32770
+# CHECK: lui $8, %hi(symbol-1)         # encoding: [0xff'A',0xff'A',0x08,0x3c]
+# CHECK:                               #   fixup A - offset: 0, value: symbol@ABS_HI, kind: fixup_Mips_HI16
+# CHECK: ori $8, $8, %lo(symbol+32766) # encoding: [0xfe'A',0x7f'A',0x08,0x35]
+# CHECK:                               #   fixup A - offset: 0, value: symbol@ABS_LO, kind: fixup_Mips_LO16
+  la $8, symbol-65538
+# CHECK: lui $8, %hi(symbol-1)         # encoding: [0xff'A',0xff'A',0x08,0x3c]
+# CHECK:                               #   fixup A - offset: 0, value: symbol@ABS_HI, kind: fixup_Mips_HI16
+# CHECK: ori $8, $8, %lo(symbol-2)     # encoding: [0xfe'A',0xff'A',0x08,0x35]
+# CHECK:                               #   fixup A - offset: 0, value: symbol@ABS_LO, kind: fixup_Mips_LO16
+  la $8, symbol-0x11111
+# CHECK: lui $8, %hi(symbol-1)         # encoding: [0xff'A',0xff'A',0x08,0x3c]
+# CHECK:                               #   fixup A - offset: 0, value: symbol@ABS_HI, kind: fixup_Mips_HI16
+# CHECK: ori $8, $8, %lo(symbol-4369)  # encoding: [0xef'A',0xee'A',0x08,0x35]
+# CHECK:                               #   fixup A - offset: 0, value: symbol@ABS_LO, kind: fixup_Mips_LO16
 
 # LW/SW and LDC1/SDC1 of symbol address, done by MipsAsmParser::expandMemInst():
   .set noat
   lw $10, symbol($4)
 # CHECK: lui     $10, %hi(symbol)        # encoding: [A,A,0x0a,0x3c]
 # CHECK:                                 #   fixup A - offset: 0, value: symbol@ABS_HI, kind: fixup_Mips_HI16
 # CHECK: addu    $10, $10, $4            # encoding: [0x21,0x50,0x44,0x01]
 # CHECK: lw      $10, %lo(symbol)($10)   # encoding: [A,A,0x4a,0x8d]