Re: [Qemu-devel] [PATCH 11/15] tcg-mips: Use mips64r6 instructions in tcg_out_movi

[James Hogan]

Hi Richard,

On Tue, Feb 09, 2016 at 04:50:52PM +0000, James Hogan wrote:
> > @@ -589,6 +608,50 @@ static void tcg_out_movi(TCGContext *s, TCGType type,
> >      }
> >      if (TCG_TARGET_REG_BITS == 32 || arg == (int32_t)arg) {
> >          tcg_out_opc_imm(s, OPC_LUI, ret, TCG_REG_ZERO, arg >> 16);
> > +    } else if (use_mips32r6_instructions) {
> > +        tcg_target_long disp = arg - (intptr_t)s->code_ptr;
> > +        if (disp == sextract32(disp, 2, 19) * 4) {
> > +            tcg_out_opc_pc19(s, OPC_ADDIUPC, ret, disp >> 2);
> > +            return;
> > +        } else if ((disp & ~(tcg_target_long)0xffff)
> > +                   == sextract32(disp, 16, 16) * 0x10000) {
> > +            tcg_out_opc_imm(s, OPC_ALUIPC, ret, 0, disp >> 16);
> 
> I think ret and 0 are the wrong way around here. You're putting 0 in rs
> (the destination register), which causes a seg fault.
> 
> OUT: [size=56] 
> 0xfff30b0064:  lw       s1,-8(s0) 
> 0xfff30b0068:  bnezalc  zero,s1,0xfff30b0090 
> 0xfff30b006c:  nop 
> 0xfff30b0070:  j        0xfff0000000 
> 0xfff30b0074:  nop 
> 0xfff30b0078:  lui      s1,0xbfc0 
> 0xfff30b007c:  ori      s1,s1,0x580 
> 0xfff30b0080:  sd       s1,256(s0) 
> 0xfff30b0084:  aluipc   zero,0xfeb7 
> 0xfff30b0088:  j        0xfff30b0034 
> 0xfff30b008c:  ori      v0,v0,0x4010 
> 0xfff30b0090:  aluipc   zero,0xfeb7 
> 0xfff30b0094:  j        0xfff30b0034 
> 0xfff30b0098:  ori      v0,v0,0x4013

Actually, still not quite right.

ALUIPC does
dest <- ~0xffff & (PC + sign_extend(imm16<<16))

which is effectively
dest <- PC & ~0xffff + sign_extend(imm16<<16)

so disp should be between arg and code_ptr & ~0xffff, i.e. something
like this I think:

diff --git a/tcg/mips/tcg-target.c b/tcg/mips/tcg-target.c
index 8205ea4e159f..9a5d31478797 100644
--- a/tcg/mips/tcg-target.c
+++ b/tcg/mips/tcg-target.c
@@ -666,12 +666,13 @@ static void tcg_out_movi(TCGContext *s, TCGType type,
         tcg_out_opc_imm(s, OPC_LUI, ret, TCG_REG_ZERO, arg >> 16);
     } else if (use_mips32r6_instructions) {
         tcg_target_long disp = arg - (intptr_t)s->code_ptr;
+        tcg_target_long disphi = arg - ((intptr_t)s->code_ptr & 
~(tcg_target_long)0xffff);
         if (disp == sextract32(disp, 2, 19) * 4) {
             tcg_out_opc_pc19(s, OPC_ADDIUPC, ret, disp >> 2);
             return;
-        } else if ((disp & ~(tcg_target_long)0xffff)
-                   == sextract32(disp, 16, 16) * 0x10000) {
-            tcg_out_opc_imm(s, OPC_ALUIPC, 0, ret, disp >> 16);
+        } else if ((disphi & ~(tcg_target_long)0xffff)
+                   == sextract32(disphi, 16, 16) * 0x10000) {
+            tcg_out_opc_imm(s, OPC_ALUIPC, 0, ret, disphi >> 16);
         } else {
             TCGReg in = TCG_REG_ZERO;
             tcg_target_long tmp = (int16_t)arg;

Otherwise, in this case its trying to load the immediate 0xfff1c30000
relative to 0xfff30b0084, and calculates a disp of FEB7FF7C, which is
truncated to 0xFEB7. The result is then:
0xfff30b0000 + (int)0xfeb70000 = 0xfff1c20000
which is off by 64KiB.

With the above change we get:
disphi = 0xfeb80000
and the result is then:
0xfff30b0000 + (int)0xfeb80000 = 0xfff1c30000

Cheers
James

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