From: Greta Yorsh Date: Fri, 5 Apr 2013 17:13:54 +0000 (+0100) Subject: arm.md: Comment on splitting Thumb1 patterns. X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=2385b218b2157281f844a5f58239f15a6db49a33;p=gcc.git arm.md: Comment on splitting Thumb1 patterns. 2013-04-05 Greta Yorsh * config/arm/arm.md: Comment on splitting Thumb1 patterns. From-SVN: r197529 --- diff --git a/gcc/ChangeLog b/gcc/ChangeLog index 2bd27c3bf3a..9e0e5c9542f 100644 --- a/gcc/ChangeLog +++ b/gcc/ChangeLog @@ -1,3 +1,7 @@ +2013-04-05 Greta Yorsh + + * config/arm/arm.md: Comment on splitting Thumb1 patterns. + 2013-04-05 Greta Yorsh * config/arm/arm.md (arm_smax_insn): Convert define_insn into diff --git a/gcc/config/arm/arm.md b/gcc/config/arm/arm.md index 68519f42711..a1789a27c44 100644 --- a/gcc/config/arm/arm.md +++ b/gcc/config/arm/arm.md @@ -22,6 +22,25 @@ ;;- See file "rtl.def" for documentation on define_insn, match_*, et. al. +;; Beware of splitting Thumb1 patterns that output multiple +;; assembly instructions, in particular instruction such as SBC and +;; ADC which consume flags. For example, in the pattern thumb_subdi3 +;; below, the output SUB implicitly sets the flags (assembled to SUBS) +;; and then the Carry flag is used by SBC to compute the correct +;; result. If we split thumb_subdi3 pattern into two separate RTL +;; insns (using define_insn_and_split), the scheduler might place +;; other RTL insns between SUB and SBC, possibly modifying the Carry +;; flag used by SBC. This might happen because most Thumb1 patterns +;; for flag-setting instructions do not have explicit RTL for setting +;; or clobbering the flags. Instead, they have the attribute "conds" +;; with value "set" or "clob". However, this attribute is not used to +;; identify dependencies and therefore the scheduler might reorder +;; these instruction. Currenly, this problem cannot happen because +;; there are no separate Thumb1 patterns for individual instruction +;; that consume flags (except conditional execution, which is treated +;; differently). In particular there is no Thumb1 armv6-m pattern for +;; sbc or adc. + ;;--------------------------------------------------------------------------- ;; Constants