[IR] redefine 'UnsafeAlgebra' / 'reassoc' fast-math-flags and add 'trans' fast-math-flag

As discussed on llvm-dev:
http://lists.llvm.org/pipermail/llvm-dev/2016-November/107104.html
and again more recently:
http://lists.llvm.org/pipermail/llvm-dev/2017-October/118118.html

...this is a step in cleaning up our fast-math-flags implementation in IR to better match
the capabilities of both clang's user-visible flags and the backend's flags for SDNode.

As proposed in the above threads, we're replacing the 'UnsafeAlgebra' bit (which had the 
'umbrella' meaning that all flags are set) with a new bit that only applies to algebraic 
reassociation - 'AllowReassoc'.

We're also adding a bit to allow approximations for library functions called 'ApproxFunc' 
(this was initially proposed as 'libm' or similar).

...and we're out of bits. 7 bits ought to be enough for anyone, right? :) FWIW, I did 
look at getting this out of SubclassOptionalData via SubclassData (spacious 16-bits), 
but that's apparently already used for other purposes. Also, I don't think we can just 
add a field to FPMathOperator because Operator is not intended to be instantiated. 
We'll defer movement of FMF to another day.

We keep the 'fast' keyword. I thought about removing that, but seeing IR like this:
%f.fast = fadd reassoc nnan ninf nsz arcp contract afn float %op1, %op2
...made me think we want to keep the shortcut synonym.

Finally, this change is binary incompatible with existing IR as seen in the 
compatibility tests. This statement:
"Newer releases can ignore features from older releases, but they cannot miscompile 
them. For example, if nsw is ever replaced with something else, dropping it would be 
a valid way to upgrade the IR." 
( http://llvm.org/docs/DeveloperPolicy.html#ir-backwards-compatibility )
...provides the flexibility we want to make this change without requiring a new IR 
version. Ie, we're not loosening the FP strictness of existing IR. At worst, we will 
fail to optimize some previously 'fast' code because it's no longer recognized as 
'fast'. This should get fixed as we audit/squash all of the uses of 'isFast()'.

Note: an inter-dependent clang commit to use the new API name should closely follow 
commit.

Differential Revision: https://reviews.llvm.org/D39304



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@317488 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 28 insertions, 4 deletions

diff --git a/test/Assembler/fast-math-flags.ll b/test/Assembler/fast-math-flags.ll
index 4ef3607e1d0..664b1bd271e 100644
--- a/test/Assembler/fast-math-flags.ll
+++ b/test/Assembler/fast-math-flags.ll
@@ -7,6 +7,8 @@
 @vec    = external global <3 x float>
 @arr    = external global [3 x float]
 
+declare float @foo(float)
+
 define float @none(float %x, float %y) {
 entry:
 ; CHECK:  %vec = load  <3 x float>,  <3 x float>* @vec
@@ -86,6 +88,28 @@ entry:
   ret float %c
 }
 
+; CHECK: @reassoc(
+define float @reassoc(float %x, float %y) {
+; CHECK: %a = fsub reassoc float %x, %y
+  %a = fsub reassoc float %x, %y
+; CHECK: %b = fmul reassoc float %x, %y
+  %b = fmul reassoc float %x, %y
+; CHECK: %c = call reassoc float @foo(float %b)
+  %c = call reassoc float @foo(float %b)
+  ret float %c
+}
+
+; CHECK: @afn(
+define float @afn(float %x, float %y) {
+; CHECK: %a = fdiv afn float %x, %y
+  %a = fdiv afn float %x, %y
+; CHECK: %b = frem afn float %x, %y
+  %b = frem afn float %x, %y
+; CHECK: %c = call afn float @foo(float %b)
+  %c = call afn float @foo(float %b)
+  ret float %c
+}
+
 ; CHECK: no_nan_inf
 define float @no_nan_inf(float %x, float %y) {
 entry:
@@ -130,10 +154,10 @@ entry:
 ; CHECK:  %arr    = load [3 x float], [3 x float]* @arr
   %arr    = load [3 x float], [3 x float]* @arr
 
-; CHECK:  %a = fadd nnan ninf float %x, %y
-  %a = fadd ninf nnan float %x, %y
-; CHECK:  %a_vec = fadd nnan <3 x float> %vec, %vec
-  %a_vec = fadd nnan <3 x float> %vec, %vec
+; CHECK:  %a = fadd nnan ninf afn float %x, %y
+  %a = fadd ninf nnan afn float %x, %y
+; CHECK:  %a_vec = fadd reassoc nnan <3 x float> %vec, %vec
+  %a_vec = fadd reassoc nnan <3 x float> %vec, %vec
 ; CHECK:  %b = fsub fast float %x, %y
   %b = fsub nnan nsz fast float %x, %y
 ; CHECK:  %b_vec = fsub nnan <3 x float> %vec, %vec