<div dir="ltr">The problem here is that doing this can introduce a race which is undefined at the IR level.<div>In the example you gave above I suspect that this is a bug in GCC. I may have missed something in the assembly, but it appears that gcc loads a copy of globalvar in w3, does stuff with w3 and then stores w3 in globalvar. This is unsound in the case where the loop is run with n = 0.</div>
<div>For an example, if you have a thread run foo(0,0) in a loop (so not doing anything) and another thread doing:</div><div>for (int i = 0 ; i <1000000 ; ++i) {</div><div> globalvar = i;</div><div> assert(globalvar == i);</div>
<div>}</div><div>the code should never assert. But if you host the load/store out of the loop (as GCC appears to do), then occasionaly there will be something like</div><div>thread 2: globalvar = i (= 42)</div><div>thread 1: w3 = globalvar (= 42)</div>
<div>thread 2: ++i</div><div>thread 2: globalvar = i (= 43)</div><div>thread 1: globalvar =w3 (= 42)</div><div>thread 2: assert(globalvar == i) </div><div>And it will assert (42 == 43) and crash.</div><div><br></div><div>
Shorter answer:</div><div>- speculatively executing stores is unsound because the value may have been modified behind your back by another thread.</div><div>- speculatively executing loads might not be observable in some specific case but may always introduce races, thus introducing undefined behaviour and breaking assumptions that other passes may rely on.</div>
<div><br></div><div>Best regards,</div><div>Robin</div></div><div class="gmail_extra"><br><br><div class="gmail_quote">On Tue, Sep 2, 2014 at 3:46 PM, Balaram Makam <span dir="ltr"><<a href="mailto:bmakam@codeaurora.org" target="_blank">bmakam@codeaurora.org</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">All,<br>
<br>
If we can speculatively execute a load instruction, why isn’t it safe to hoist it out by promoting it to a scalar in LICM pass?<br>
<br>
<br>
There is a comment in LICM pass that if a load/store is conditional then it is not safe because it would break the LLVM concurrency model (See commit 73bfa4a).<br>
It has an IR test for checking this in test/Transforms/LICM/scalar-promote-memmodel.ll<br>
<br>
However, I have a sample code where GCC is able to promote the memory to scalar and hoist/sink load/store out of loop but LLVM cannot.<br>
Is GCC being aggressive here or LLVM missing out an opportunity?<br>
<br>
Here is my sample code:<br>
<br>
extern int globalvar;<br>
void foo(int n , int incr) {<br>
unsigned int i;<br>
for (i = 0 ; i < n; i += incr ) {<br>
if (i < n/2)<br>
globalvar += incr;<br>
}<br>
return;<br>
}<br>
<br>
GCC output:<br>
<br>
$ aarch64-linux-gnu-g++ -S -o - -O3 -ffast-math -march=armv8-a+simd test.cpp<br>
.arch armv8-a+fp+simd<br>
.file "test.cpp"<br>
.text<br>
.align 2<br>
.global _Z3fooii<br>
.type _Z3fooii, %function<br>
_Z3fooii:<br>
.LFB0:<br>
.cfi_startproc<br>
cbz w0, .L1<br>
adrp x6, globalvar<br>
add w5, w0, w0, lsr 31<br>
ldr w3, [x6,#:lo12:globalvar] <== hoist load of globalvar<br>
mov w2, 0<br>
asr w5, w5, 1<br>
.L4:<br>
cmp w5, w2<br>
add w2, w2, w1<br>
add w4, w3, w1<br>
csel w3, w4, w3, hi<br>
cmp w2, w0<br>
bcc .L4<br>
str w3, [x6,#:lo12:globalvar] <== sink store of globalvar<br>
.L1:<br>
ret<br>
.cfi_endproc<br>
.LFE0:<br>
.size _Z3fooii, .-_Z3fooii<br>
.ident "GCC: (crosstool-NG linaro-1.13.1-4.8-2014.01 - Linaro GCC 2013.11) 4.9.0"<br>
<br>
LLVM output:<br>
<br>
$ clang-aarch64-x++ -S -o - -O3 -ffast-math -fslp-vectorize test.cpp<br>
.text<br>
.file "test.cpp"<br>
.globl _Z3fooii<br>
.align 2<br>
.type _Z3fooii,@function<br>
_Z3fooii: // @_Z3fooii<br>
// BB#0: // %entry<br>
cbz w0, .LBB0_5<br>
// BB#1: // %<a href="http://for.body.lr.ph" target="_blank">for.body.lr.ph</a><br>
mov w8, wzr<br>
cmp w0, #0 // =0<br>
cinc w9, w0, lt<br>
asr w9, w9, #1<br>
adrp x10, globalvar<br>
.LBB0_2: // %for.body<br>
// =>This Inner Loop Header: Depth=1<br>
cmp w8, w9<br>
b.hs .LBB0_4<br>
// BB#3: // %if.then<br>
// in Loop: Header=BB0_2 Depth=1<br>
ldr w11, [x10, :lo12:globalvar] <===== load inside loop<br>
add w11, w11, w1<br>
str w11, [x10, :lo12:globalvar] <==== store inside loop<br>
.LBB0_4: // %for.inc<br>
// in Loop: Header=BB0_2 Depth=1<br>
add w8, w8, w1<br>
cmp w8, w0<br>
b.lo .LBB0_2<br>
.LBB0_5: // %for.end<br>
ret<br>
.Ltmp1:<br>
.size _Z3fooii, .Ltmp1-_Z3fooii<br>
<br>
<br>
.ident "clang version 3.6.0 "<br>
<br>
Thanks,<br>
Balaram<br>
<br>
<br>
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</blockquote></div><br></div>