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From: "carrot at google dot com" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug target/45335] New: Use ldrd to load two consecutive words Date: Thu, 19 Aug 2010 07:18:00 -0000 [thread overview] Message-ID: <bug-45335-17659@http.gcc.gnu.org/bugzilla/> (raw) Compile the following code with options -march=armv7-a -mthumb -O2 -fpic struct S { void* p1; void* p2; void* p3; void* p4; }; void foo1(struct S* fp, struct S* otherSaveArea) { struct S* saveArea = fp - 1; printf("StackSaveArea for fp %p [%p/%p]:\n", fp, saveArea, otherSaveArea); printf(" prevFrame=%p savedPc=%p meth=%p curPc=%p fp[0]=0x%08x\n", saveArea->p1, saveArea->p2,saveArea->p3,saveArea->p4,*(unsigned int*)fp); } GCC 4.6 generates foo1: push {r4, r5, r6, lr} mov r4, r0 ldr r5, .L2 sub sp, sp, #8 mov r3, r1 sub r2, r0, #16 .LPIC0: add r5, pc mov r1, r0 mov r0, r5 bl printf(PLT) ldr r6, [r4, #-4] // A ldr r5, [r4, #0] // B ldr r0, .L2+4 ldr r1, [r4, #-16] // C ldr r2, [r4, #-12] // D .LPIC1: add r0, pc ldr r3, [r4, #-8] str r6, [sp, #0] // E str r5, [sp, #4] // F bl printf(PLT) add sp, sp, #8 pop {r4, r5, r6, pc} Notice instructions A and B loading two consecutive words, they can be written as a single instruction ldrd r6, r5, [r4, -4] It results in shorter and potentially faster code. Similarly instructions C and D can be replace by a ldrd. Instructions E and F shows a chance for strd instruction. We don't have a ldrd insn pattern to represent two separate loads. Maybe we can define one and let combine to find the chances. There is also the same opportunity for ARM instructions, but there are more constraints on register usage. In arm mode, the first register must be even numbered and the second register number must be exact successor of the first one. These are similar to the constraints of ldm and stm, and our register allocator can't handle them. -- Summary: Use ldrd to load two consecutive words Product: gcc Version: 4.6.0 Status: UNCONFIRMED Severity: enhancement Priority: P3 Component: target AssignedTo: unassigned at gcc dot gnu dot org ReportedBy: carrot at google dot com GCC build triplet: i686-linux GCC host triplet: i686-linux GCC target triplet: arm-eabi http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45335
next reply other threads:[~2010-08-19 7:18 UTC|newest] Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top 2010-08-19 7:18 carrot at google dot com [this message] 2010-08-20 3:13 ` [Bug target/45335] " carrot at google dot com 2010-08-23 8:24 ` ramana at gcc dot gnu dot org
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