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Re: [Qemu-devel] [patch] make qemu work with GCC 4

From: Thiemo Seufer
Subject: Re: [Qemu-devel] [patch] make qemu work with GCC 4
Date: Thu, 30 Aug 2007 21:28:11 +0100
User-agent: Mutt/1.5.16 (2007-06-11)

Michael Matz wrote:
> Hi,
> [please keep me CCed, I'm not on this list]
> the below patch let's qemu be compiled by GCC 4.2 (probably also 4.1 and 
> others) for most hosts (i386,x86_64,ia64,ppc).  s390 as host is missing, 
> and needs a compiler change to emit the literal store inline again, as the 
> literal pool at the end fundamentally breaks the assumption that qemu can 
> paste together the code snippets by patching out the return.  I have no 
> HOST_{ARM,MIPS*,ALPHA,SPARC*,M68K} machines to compile for that.
> It specifically changes these things:
> * ppc: adds -fno-section-anchors to OP_CFLAGS, as dyngen isn't prepared
>        to deal with the relocs resulting from using section anchors

Maybe this should be handled more generally then, not ppc specific, like
other "offending" compiler options: check if the compiler knows the
option, if yes, disable the feature.

> * ppc: on target-alpha op_reset_FT GCC4 uses a floating point constant 0.0
>        to reset the ft regs, which in turn is loaded from the data 
>        section.  The reloc for that is unhandled.  Using -ffast-math would 
>        work around this, but I chose to be conservative and change only
>        the op.c snippet in question.  See the comment there.
> * i386: well, most of you will know that GCC4 doesn't compile qemu because 
>        of reload.  The inherent problem is, that qemu uses 64bit
>        entities in some places (sometimes structs), which GCC (4.x) 
>        manages to place in registers, i.e. needs 2 hardregs.  But it 
>        sometimes just so happens that an instruction needing such DImode
>        reg also has a memory operand with an indexed address (reg plus 
>        reg), hence two hardregs more.  But qemu by default leaves just 
>        three free registers for compiling op.c --> boom.  This is somewhat 
>        hard to work around in GCC (trust me :) ).
>        I solved that by placing one of the T[012] operands into memory
>        for HOST_I386, thereby freeing one reg.  Here's some justification 
>        of why that doesn't really cost performance: with three free regs
>        GCC is already spilling like mad in the snippets, we just trade one
>        of those memory accesses (to stack) with one other mem access to 
>        the cpu_state structure, which will be in cache.

Could you back up this assumption with some numbers? :-)
If there is a significant difference I recommend to make that workaround
conditional on GCC4 as well as HOST_I386.

> diff -urp qemu-0.9.0.cvs.orig/target-arm/cpu.h qemu-0.9.0.cvs/target-arm/cpu.h
> --- qemu-0.9.0.cvs.orig/target-arm/cpu.h      2007-06-24 14:09:48.000000000 
> +0200
> +++ qemu-0.9.0.cvs/target-arm/cpu.h   2007-08-21 21:38:36.000000000 +0200
> @@ -52,6 +52,9 @@ typedef uint32_t ARMReadCPFunc(void *opa
>   */
>  typedef struct CPUARMState {
> +#if defined(HOST_I386)
> +    uint32_t t1;
> +#endif
>      /* Regs for current mode.  */
>      uint32_t regs[16];
>      /* Frequently accessed CPSR bits are stored separately for efficiently.
> diff -urp qemu-0.9.0.cvs.orig/target-arm/exec.h 
> qemu-0.9.0.cvs/target-arm/exec.h
> --- qemu-0.9.0.cvs.orig/target-arm/exec.h     2007-06-03 19:44:36.000000000 
> +0200
> +++ qemu-0.9.0.cvs/target-arm/exec.h  2007-08-21 21:48:48.000000000 +0200
> @@ -23,7 +23,12 @@
>  register struct CPUARMState *env asm(AREG0);
>  register uint32_t T0 asm(AREG1);
>  register uint32_t T1 asm(AREG2);
> +#ifndef HOST_I386
>  register uint32_t T2 asm(AREG3);
> +#else
> +#define T2 (env->t1)
> +#endif

T2/t1 mismatch, it seems. Likewise for mips and ppc.


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