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[Qemu-devel] [PATCH 2/2] tcg/optimize: fix constant signedness
From: |
Aurelien Jarno |
Subject: |
[Qemu-devel] [PATCH 2/2] tcg/optimize: fix constant signedness |
Date: |
Fri, 10 Jul 2015 18:03:31 +0200 |
By convention, on a 64-bit host TCG internally stores 32-bit constants
as sign-extended. This is not the case in the optimizer when a 32-bit
constant is folded.
This doesn't seem to have more consequences than suboptimal code
generation. For instance the x86 backend assumes sign-extended constants,
and in some rare cases uses a 32-bit unsigned immediate 0xffffffff
instead of a 8-bit signed immediate 0xff for the constant -1. This is
with a ppc guest:
before
------
---- 0x9f29cc
movi_i32 tmp1,$0xffffffff
movi_i32 tmp2,$0x0
add2_i32 tmp0,CA,CA,tmp2,r6,tmp2
add2_i32 tmp0,CA,tmp0,CA,tmp1,tmp2
mov_i32 r10,tmp0
0x7fd8c7dfe90c: xor %ebp,%ebp
0x7fd8c7dfe90e: mov %ebp,%r11d
0x7fd8c7dfe911: mov 0x18(%r14),%r9d
0x7fd8c7dfe915: add %r9d,%r10d
0x7fd8c7dfe918: adc %ebp,%r11d
0x7fd8c7dfe91b: add $0xffffffff,%r10d
0x7fd8c7dfe922: adc %ebp,%r11d
0x7fd8c7dfe925: mov %r11d,0x134(%r14)
0x7fd8c7dfe92c: mov %r10d,0x28(%r14)
after
-----
---- 0x9f29cc
movi_i32 tmp1,$0xffffffffffffffff
movi_i32 tmp2,$0x0
add2_i32 tmp0,CA,CA,tmp2,r6,tmp2
add2_i32 tmp0,CA,tmp0,CA,tmp1,tmp2
mov_i32 r10,tmp0
0x7f37010d490c: xor %ebp,%ebp
0x7f37010d490e: mov %ebp,%r11d
0x7f37010d4911: mov 0x18(%r14),%r9d
0x7f37010d4915: add %r9d,%r10d
0x7f37010d4918: adc %ebp,%r11d
0x7f37010d491b: add $0xffffffffffffffff,%r10d
0x7f37010d491f: adc %ebp,%r11d
0x7f37010d4922: mov %r11d,0x134(%r14)
0x7f37010d4929: mov %r10d,0x28(%r14)
Cc: Richard Henderson <address@hidden>
Signed-off-by: Aurelien Jarno <address@hidden>
---
tcg/optimize.c | 10 +++++-----
1 file changed, 5 insertions(+), 5 deletions(-)
diff --git a/tcg/optimize.c b/tcg/optimize.c
index 18283cf..cd0e793 100644
--- a/tcg/optimize.c
+++ b/tcg/optimize.c
@@ -395,7 +395,7 @@ static TCGArg do_constant_folding(TCGOpcode op, TCGArg x,
TCGArg y)
{
TCGArg res = do_constant_folding_2(op, x, y);
if (op_bits(op) == 32) {
- res &= 0xffffffff;
+ res = (int32_t)res;
}
return res;
}
@@ -1128,8 +1128,8 @@ void tcg_optimize(TCGContext *s)
rl = args[0];
rh = args[1];
- tcg_opt_gen_movi(s, op, args, rl, (uint32_t)a);
- tcg_opt_gen_movi(s, op2, args2, rh, (uint32_t)(a >> 32));
+ tcg_opt_gen_movi(s, op, args, rl, (int32_t)a);
+ tcg_opt_gen_movi(s, op2, args2, rh, (int32_t)(a >> 32));
/* We've done all we need to do with the movi. Skip it. */
oi_next = op2->next;
@@ -1149,8 +1149,8 @@ void tcg_optimize(TCGContext *s)
rl = args[0];
rh = args[1];
- tcg_opt_gen_movi(s, op, args, rl, (uint32_t)r);
- tcg_opt_gen_movi(s, op2, args2, rh, (uint32_t)(r >> 32));
+ tcg_opt_gen_movi(s, op, args, rl, (int32_t)r);
+ tcg_opt_gen_movi(s, op2, args2, rh, (int32_t)(r >> 32));
/* We've done all we need to do with the movi. Skip it. */
oi_next = op2->next;
--
2.1.4