RE: [PATCH, MIPS] Compact branch support for MIPS32R6/MIPS64R6

["Moore, Catherine" <Catherine_Moore at mentor dot com>]

Hi Matthew,

> -----Original Message-----
> From: Matthew Fortune [mailto:Matthew.Fortune@imgtec.com]
> Sent: Wednesday, July 22, 2015 6:19 PM
> Subject: [PATCH, MIPS] Compact branch support for MIPS32R6/MIPS64R6
> 
This patch looks really good.  I have a couple of questions and a couple of nits that need to be fixed up.

> A full range of 'compact' branch instructions were introduced to MIPS
> as part of Release 6. The compact term is used to identify the fact
> that these do not have a delay slot.
> 
> So how does all this work in GCC?
> 
> Compact branches are used based on a branch policy. The polices are:
> 
> never: Only use delay slot branches
> optimal: Do whatever is best for the current architecture.  This will
>          generally mean that delay slot branches will be used if the delay
>          slot gets filled but otherwise a compact branch will be used. A
>          special case here is that JAL and J will not be used in R6 code
>          regardless of whether the delay slot could be filled.
> always: Never emit a delay slot form of a branch if a compact form exists.
>         This policy cannot apply 100% as FP branches (and MSA branches when
>         committed) only have delay slot forms.
> 
> These user choices are combined with the features available in the chosen
> architecture and, in particular, the optimal form will get handled like
> 'never' when there are no compact branches available and will get handled
> like 'always' when there are no delay slot branches available.
> 

Why did you choose to make this a user-selectable option?  Why not always generated optimal?
I don't have a strong opinion about it, but the options seem to complicate things and I'm interested in your rationale.

> 
> The most complicated aspect of this change is to the MIPS_CALL and
> MICROMIPS_J macros. These have been rewritten from scratch as a function
> that generates an instruction instead.  

Thank you for cleaning this up.  The new function is much easier to follow.
 
> diff --git a/gcc/config/mips/mips.c b/gcc/config/mips/mips.c
> index c3cd52d..c0f2884 100644
> --- a/gcc/config/mips/mips.c
> +++ b/gcc/config/mips/mips.c
> 
> +/* Return the asm template for a call.  OPERANDS are the operands,
> TARGET_OPNO
> +   is the operand number of the target.  SIZE_OPNO is the operand number
> of
> +   the argument size operand that can optionally hold the call attributes.  If
> +   SIZE_OPNO is not -1 and the call is indirect, use the function symbol from
> +   the call attributes to attach a R_MIPS_JALR relocation to the call.
> +

Might as well mention LINK_P here as well.

> +   When generating GOT code without explicit relocation operators, all calls
> +   should use assembly macros.  Otherwise, all indirect calls should use "jr"
> +   or "jalr"; we will arrange to restore $gp afterwards if necessary.  Finally,
> +   we can only generate direct calls for -mabicalls by temporarily switching
> +   to non-PIC mode.
> +
> +   For microMIPS jal(r), we try to generate jal(r)s when a 16-bit
> +   instruction is in the delay slot of jal(r).
> +
> +   Where compact branches are available, we try to use them if the delay
> slot
> +   has a NOP (or equivalently delay slots were not enabled for the instruction
> +   anyway).  */
> +
> +const char *
> +mips_output_jump (rtx *operands, int target_opno, int size_opno, bool
> link_p)
> +{
> @@ -13038,6 +13165,59 @@ mips_output_conditional_branch (rtx_insn
> *insn, rtx *operands,
>    return "";
>  }
> 
> +const char *
> +mips_output_equal_conditional_branch (rtx_insn* insn, rtx *operands,
> +				      bool inverted_p)

This function needs a comment.

diff --git a/gcc/config/mips/mips.opt b/gcc/config/mips/mips.opt
index 348c6e0..84887d1 100644
--- a/gcc/config/mips/mips.opt
+++ b/gcc/config/mips/mips.opt
@@ -418,3 +418,20 @@ Driver
 mload-store-pairs
 Target Report Var(TARGET_LOAD_STORE_PAIRS) Init(1)
 Enable load/store bonding.
+
+mcompact-branches=
+Target RejectNegative JoinedOrMissing Var(mips_cb) Report Enum(mips_cb_setting) Init(MIPS_CB_OPTIMAL)
+Specify the compact branch usage policy
+
+Enum
+Name(mips_cb_setting) Type(enum mips_cb_setting)
+Policies available for use with -mcompact-branches=:
+
+EnumValue
+Enum(mips_cb_setting) String(never) Value(MIPS_CB_NEVER)
+
+EnumValue
+Enum(mips_cb_setting) String(optimal) Value(MIPS_CB_OPTIMAL)
+
+EnumValue
+Enum(mips_cb_setting) String(always) Value(MIPS_CB_ALWAYS)

These need to be documented in invoke.texi.

diff --git a/gcc/config/mips/mips.h b/gcc/config/mips/mips.h
index 5bc562e..04fe6d0 100644
--- a/gcc/config/mips/mips.h
+++ b/gcc/config/mips/mips.h
> @@ -92,6 +92,23 @@ struct mips_cpu_info {
>  /* True if we are generating position-independent VxWorks RTP code.  */
>  #define TARGET_RTP_PIC (TARGET_VXWORKS_RTP && flag_pic)
> 
> +/* Set based on a combination of compact branch policy and ISA support.
> */
> +#define TARGET_CB_NEVER (mips_cb == MIPS_CB_NEVER	\
> +			 || (mips_cb == MIPS_CB_OPTIMAL \
> +			     && !ISA_HAS_COMPACT_BRANCHES))
> +#define TARGET_CB_MAYBE (TARGET_CB_ALWAYS		\
> +			 || (mips_cb == MIPS_CB_OPTIMAL \
> +			     && ISA_HAS_COMPACT_BRANCHES))
> +#define TARGET_CB_ALWAYS (mips_cb == MIPS_CB_ALWAYS	\
> +			 || (mips_cb == MIPS_CB_OPTIMAL \
> +			     && !ISA_HAS_DELAY_SLOTS))
> +
I would appreciate a more detailed comment here ...

@@ -871,6 +888,10 @@ struct mips_cpu_info {

 #define ISA_HAS_JR             (mips_isa_rev <= 5)

+#define ISA_HAS_DELAY_SLOTS    1

If this is a placeholder for the microMIPS patch, then OK.  Otherwise, what's the point?


My test run had only one test that executed with the -mcompact-branch= option.  That may have been because I used RUNTESTFLAGS (the problem that we discussed at Cauldron).  It looks like you put the logic in mips.exp though to have the option throw more often.  In any case, I would like to see some explicit testing of the options (if they are kept).  You've modified the tests to accept both the compact and non-compact forms.  It would be nice to ensure that the compact form is not generated when -mcompact-branch=never.

Catherine